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Evolution 
and Genetics 



THE 

MODERN 

THEORY 

OF 

EVOLUTION 



hvt 






David J. Merrell 

University of Minnesota 

With illustrations by 

OLIVIA JENSEN INGERSOLL 



HOLT, RINEHART AND WINSTON 
New York 




Copyright © 1962 by Holt, Rinehart and Winston, Inc. 
All Rights Reserved 

90123 4 9876 



Library of Congress Catalog Card Number: 62-8420 

03-010950-7 

Printed in the United States of America 






Preface 



The human species, in a remarkable manner, has be- 
come the dominant species on the earth. Man's range has ex- 
panded explosively out of the tropics, and as he has gained 
mastery over his competitors, his parasites, and his environ- 
ment, human numbers have increased at an accelerating pace. 
These biological facts are of primary significance in the world 
of today. To understand them requires an understanding of the 
evolutionary forces that have been at work in the past and con- 
tinue to work at present. 

The study of evolution received its major impetus just 
over a century ago with the publication of Darwin's Origin of 
Species in 1859. Since then, great progress has been made in 
biology. Knowledge has accumulated so rapidly that the field 
has splintered into a number of subdisciplines. As specializa- 
tion has increased, the need for a unifying principle in biology 
has grown. The most notable success in tying together the many 



VI • PREFACE 

threads of biological thought has been achieved by a return to the study of 
evolution. It is a return because, after the original impetus from Darwin's 
work had dwindled, a rather strong reaction against Darwinism developed 
early in this century. The validity of the theory of evolution was generally 
accepted by biologists, but the discoveries of the early geneticists seemed to 
cast considerable doubt on his theory of natural selection. 

A reflection of the appraisal of Darwin at that time can be found in 
Nordenskiold's History of Biology (1927). "To raise the theory of selection, 
as has often been done, to the rank of a 'natural law' comparable in value with 
the law of gravity established by Newton is, of course, quite irrational, as time 
has already shown: Darwin's theory of the origin of species was long ago 
abandoned. Other facts established by Darwin are all of second-rate value. But 
if we measure him by his influence on the general cultural development of 
humanity, then the proximity of his grave to Newton's is fully justified." How- 
ever, further progress, particularly in the fields of genetics, systematics, and 
paleontology, has led to an increased understanding not only of evolution but 
of the mechanism by which it takes place. 

The expanded theory of evolution that has been recently developed is 
sometimes known as neo-Darwinism, or as "the modern synthesis," and the 
theory of natural selection has proven to be more resilient than Darwin's critics 
supposed. Furthermore, the study of evolution has come to be a unifying force 
in biology, drawing together information from many disciplines into a com- 
prehensive and comprehensible whole. At the present time, more research on 
evolutionary problems is being conducted than at any time since 1859. 

It has become routine for biologists to preface their remarks about 
evolution with the statement, "Everyone now accepts the fact of evolution." 
However, my experience has been, in talking with a variety of audiences, that 
not everyone does accept evolution as a fact, even though they may have been 
exposed to the concept. Furthermore, since there are millions of people in many 
parts of the world who have never even heard of the theory of evolution, the 
supposition that everyone accepts it obviously needs some qualification. Because 
all mankind is caught up in the same evolutionary skein, it seems highly de- 
sirable that all of us should be made aware of this fact. If man continues to 
pursue his evolutionary future blindly, without awareness or regard for the 
forces at work, that future may be bedeviled by unnecessary hazards and hard- 
ships. 

Doubts about the validity of evolution by intelligent and supposedly 
educated people are due in many cases to the fact that they have never really 
heard the evidence in its favor. The extent of this ignorance was most forcibly 
impressed on me during a recent talk with a group of high school biology 
teachers. As the discussion progressed, it became clear that at least half of this 
select group of teachers did not themselves believe in evolution, and thus it 
was a real problem for them to decide how to handle the subject in class. Their 



PREFACE • Vll 

reactions made clear the need to continue presenting the case for evolution to 
new generations of students. For unless students, at some stage in their training, 
are given the opportunity to become acquainted with the nature, variety, and 
weight of the data concerning evolution, they are required to accept evolution 
on faith. It is, therefore, perhaps not surprising that people never exposed to 
the evidence may find other explanations more emotionally satisfying. Only 
after the facts have been reviewed and understood does the theory of evolution 
become inescapable. I have no particular desire to convert anyone to a belief 
in evolution, but at the same time I feel that even those who are unconvinced 
about evolution should be familiar with the evidence. At least then their beliefs 
will not be based on ignorance, and they will know exactly what it is that they 
do not believe. 

For these reasons the first part of the book has been devoted to a 
consideration of the nature of the evidence for evolution. No more than a 
sampling of the wealth of material, of course, can be presented. It is hoped 
that for the great majority of readers this presentation will be sufficiently con- 
vincing. For those who remain in doubt as to the reality of evolution, the 
references open a number of paths from which any who follow them with an 
open mind can scarcely return unconvinced that evolution has occurred. The 
supplementary reading suggested at the end of each chapter can thus serve the 
dual purpose of documenting statements in the text and also of giving addi- 
tional information to the student desirous of learning more about a particular 
topic. I have referred freely to the writings and opinions of many authors 
without citing or documenting the actual sources in the text. Since literature 
citations and footnotes can be a major source of distraction to the weak-willed 
reader, it seemed desirable to keep such diversions to a minimum so that the 
reader will be better able to follow the argument being presented. To those not 
cited, and thereby slighted, my apologies. The references, though not complete, 
should be a sufficient guide into any area in which further information or 
documentation is desired. 

This book has been written primarily for those who wish to know 
more about the theory of evolution and the operation of evolutionary forces. 
The problem of discussing evolution is complicated by the fact that it must 
be taken up a piece at a time and fitted together like a jig-saw puzzle. Only 
when all of the pieces are together can the whole picture be fully appreciated. 
The discussion ranges over a wide variety of subjects, but an effort has been 
made to develop each topic in such a way that the reader can follow the argu- 
ment with only a minimum of background. One of the major hurdles for the 
student of biology is the number of new terms that constantly appear. If he 
does not learn the vocabulary so that he can handle the biologists' jargon, he 
remains biologically illiterate. As an aid over this hurdle, terms are generally 
explained when first introduced, but a glossary is also included at the end for 
quick reference. 



Vlll • PREFACE 

The modern theory of the mechanism of evolution is a genetic theory. 
Since without some understanding of genetics the modern theory of evolution is 
incomprehensible, it is essential to devote a section of the book to the funda- 
mental principles of genetics. From the text the reader should be able to gain 
an understanding of the basic genetic principles, but if he becomes interested 
in pursuing the subject further, he should refer to the numerous excellent books 
in the field. Evolution is a population phenomenon and is best understood in 
terms of the genetics of populations. Population genetics requires the use of 
some mathematics, which, unfortunately, causes consternation for some stu- 
dents. However, only rather simple examples have been included, requiring at 
most a knowledge of elementary algebra. A dash of common sense and a little 
persistence in dealing with this material will be well rewarded in terms of the 
insight gained. 

A biological approach has been used throughout the book, and no 
attempt has been made to explore the philosophical or religious implications of 
the theory of evolution. This approach is sometimes disturbing to students. 
However, just among the various Christian denominations, attitudes range from 
unqualified acceptance to complete rejection of evolution. Because of the diver- 
sity of opinion and belief, generalizations are virtually meaningless, and it seems 
wisest to encourage each student to reconcile his knowledge of evolution with 
his personal beliefs, if this is necessary. 

I wish to acknowledge the inspiration of Dr. Dwight E. Minnich, who 
first encouraged me to undertake teaching a course in evolution, and of the 
many students whose interest has made this particular course such a pleasure to 
teach. The comments and suggestions of my colleagues at the University of 
Minnesota, James C. Underhill, Joseph G. Gall, John W. Hall, and Frank G. 
Nordlie, have been most helpful, but I, of course, am solely responsible for 
the final form of the book. In a work of this sort, covering as it does subjects 
ranging from the origin of life to cultural anthropology, choices must be made 
in matters of emphasis and interpretation. It is hoped that the net result is a 
reasonably balanced account of current thought on evolution. 

My collaboration with Mrs. Olivia Jensen Ingersoll, whose imaginative 
drawings illustrate the book, of necessity was carried on at long range since 
her home is in Ohio. However, her competence, both as an illustrator and as 
a zoologist, greatly eased the problems involved. Finally, I wish to acknowledge 
the devoted assistance of my wife, Jessie, who assumed the onerous task of 
typing the manuscript. 

D. J. M. 
Minneapolis, Minnesota 
January, 1962 



Credits 



The following illustrations are used with the kind permission of the 
authors and publishers listed below. 

Fig. 1-2. Cott, H. B., 1940, Adaptive coloration in animals, Methuen and Co., 
Ltd. 

Fig. 4-2. Simpson, G. G., 1951, Horses, Oxford University Press. 

Fig. 8-1. The quail were very kindly made available by Dr. Dwain Warner, 
Curator of Birds, University of Minnesota Museum of Natural History. 

Fig. 12-1. Baldwin, E., 1949, An introduction to comparative biochemistry. 
Cambridge University Press. (Redrawn) 

Fig. 13-2. Lemche, H., 1957. "A new living deep-sea mollusc of the Cambro- 

Devonian class Monoplacophora," Nature 179(1) :415. 
Fig. 13-4. Ralph Buchsbaum. 

Fig. 14-2. Fuller, H. B., and O. Tippo, 1949, College botany, Holt, Rinehart 
and Winston, Inc. 

Fig. 17-1. Snyder, L. H., and P. R. David, 1957, The principles of heredity, 
5th ed., D. C. Heath and Company. 

Fig. 17-2. Srb, A., and R. D. Owen, 1952, General genetics, W. H. Free- 
man and Company. 

Fig. 18-3. Wilson, C L., and W. E. Loomis, 1957, Botany, rev. ed., Holt, 
Rinehart and Winston, Inc. 

ix 



X • CREDITS 

Fig. 21-1. Edmund Bert Gerard, Cinematographer, Great Neck, N. Y. 

Fig. 23-2. Clausen, J., and W. M. Hiesey, 1958, Experimental studies on the 
nature of species, IV, Carnegie Institution of Washington. 

Fig. 23-3. Miintzing, A., 1930, "Uber Chromosomen-vermehrung in Gale- 
op sis — Kreuzungen und ihre phylogenetische Bedeutung," Hereditas 
14:155. 

Fig. 25-1. Snyder, L. H., and P. R. David, 1957, The principles of heredity, 
5th ed., D. C. Heath and Company. (Pictures from The Cattleman) 

Fig. 28-1. Clausen, J., D. D. Keck, and W. M. Hiesey, 1947, "Heredity of 
geographically and ecologically isolated races," Am. Naturalist 81:114- 
123. 

Fig. 28-2. Moore, J. A., 1949, "Patterns of evolution in the genus Rana." 
In Genetics, paleontology, and evolution, Jepsen, G. L., E. Mayr, and 
G. G. Simpson, eds., Princeton University Press. 

Fig. 29-2. Anderson, E., 1949, Introgressive hybridization, John Wiley and 

Sons. 
Fig. 29-3. Manton, I., 1934, "The problem of Biscutella laevigata," L. 

Zeitschr. f. ind. Abst. n. Vererbungsl. 67, Springer- Verlag, Heidelberg. 

Fig. 31-4. Lack, D., 1947, Darwin's finches, Cambridge University Press. 

Fig. 32-1. Begg, C. M. M., 1959, Introduction to genetics, The Macmillan Com- 
pany. 

Fig. 32-2. Stern, C, 1954, "Two or three bristles," Am. Sci. 42:284. 

Fig. 32-3. Snyder, L. H., and P. R. David, 1957, The principles of heredity, 
5th ed., D. C Heath and Company. (Photograph by Dr. L. V. Domm) 

Fig. 33-1. a, d, and e, Zoological Society of London, b, Walker, E. P., 1954, 
The monkey book, The Macmillan Company, c, Chicago Zoological 
Park, Brookfield, 111. 

Fig. 33-2. a and b, Walker, E. P., 1954, The monkey book, The Macmillan 
Company, c, National Zoological Park, Smithsonian Institution, Wash- 
ington, D. C. 

Figs. 33-6 and 33-8. Washburn, S. L., I960, "Tools and human evolution," 
Sci. American 203(3) September I960. 

Fig. 33-7. a-e, Peabody Museum, Harvard University. 

Fig. 34-1. Begg, C. M. M., 1959, Introduction to genetics, The Macmillan Com- 
pany. 

Fig. 34-2. Sax, K., 1950. "The effects of x-rays on chromosome structure," 
/. Cell. Comp. Physiol. 35, Suppl. 1. 

Fig. 35-1. Sax, K., 1955, Standing room only, Beacon Press. 

Fig. 35-2. World population and resources, 1955, P. E. P. 16, Queen Anne's 
Gate, London. 

Fig. 35-3. Van Loon, H. W., 1932, Van Loon's geography, Simon and 
Schuster, Inc. 



Contents 



chapter i Adaptation 

TYPES OF ADAPTATION 4 

THE ENVIRONMENT 5 

ADAPTATION IN THE FROG 6 

PROTECTIVE COLORATION 10 

ADAPTATION IN MAN 12 

chapter 2 Evolutionary Thought before 
Darwin 

GREEK THOUGHT 14 

THE DECLINE OF SCIENCE 16 

THE RENAISSANCE 17 

THE NATURAL PHILOSOPHERS 17 

BIOLOGICAL RESEARCH AND WRITINGS 



PART I 

Introduction 
3 



14 



18 



XI 



xil • CONTENTS 

chapter 3 Darwin and after Darwin 25 



PART // 
The Evidence for Evolution 
chapter 4 The Fossil Record 39 

RECONSTRUCTING THE PAST 40 

EXTINCTION AND EVOLUTION 42 

VERTEBRATE EVOLUTION 44 

EVOLUTION OF THE HORSE 46 



CHAPTER 5 The Origin of the Earth and of 
the Universe 



51 



AGE OF THE UNIVERSE 51 

NATURE OF THE UNIVERSE 



53 



CHAPTER 6 The Origin of Life 57 

SPONTANEOUS GENERATION 57 

THE COMPOSITION OF LIVING THINGS 60 

FORMATION OF ORGANIC COMPOUNDS 6l 

SOURCES OF ENERGY AND FOOD 64 

chapter 7 Geographical Distribution 68 

BIOGEOGRAPHICAL REALMS 69 

PRIMITIVE AND MODERN MAMMALS IN 

THE NEOTROPICAL 72 

NEARCTIC AND PALEARCTIC 73 

RELICT ALPINE POPULATIONS 73 

PRIMITIVE SOUTHERN FAUNA 74 

CONTINENTAL AND OCEANIC ISLANDS 75 



79 



chapter 8 Systematics . . 

CLASSIFICATION 

VARIATION 80 

THE BINOMIAL SYSTEM 81 

SOME TAXONOMIC PROBLEMS 



79 



84 



chapter 9 Comparative Embryology 87 

VON baer's dicta 88 

MODIFICATIONS OF DEVELOPMENT 90 



CONTENTS • Xlll 

chapter 10 Comparative Anatomy 95 

HOMOLOGY AND ANALOGY 95 

HOMOLOGIES IN VERTEBRATES 97 

GENETIC HOMOLOGY 100 

VESTIGIAL ORGANS 101 

CHAPTER ii Comparative Biochemistry 103 

PLANT PIGMENTS 105 

PHOTORECEPTORS 106 

IMMUNOLOGY 110 

CHAPTER 12 Biochemical Adaptation 113 

AQUATIC LIFE 114 

TERRESTRIAL LIFE 117 

NITROGEN EXCRETION 120 

chapter 13 Evolution in Animals 123 

PROTOZOA 124 

PORIFERA 125 

MESOZOA 128 

COELENTERATA 128 

CTENOPHORA 129 

PLATYHELMINTHES 129 

ORIGIN OF THE METAZOA 130 

NEMERTEA 132 

ACANTHOCEPHALA 132 

NEMATODA 133 

NEMATOMORPHA, KINORHYNCHA, 

AND PRIAPULIDA 133 

GASTROTRICHA AND ROTIFERA 
ENTOPROCTA AND ECTOPROCTA 
BRACHIOPODA AND PHORONIDA 
MOLLUSCA 135 

ANNELIDA 136 

SIPUNCULIDA AND ECHIURIDA 
ONYCOPHORA, A LIVING LINK 
ARTHROPODA 139 

CHAETOGNATHA AND POGONOPHORA 139 

ECHINODERMATA 140 

HEMICHORDATA l4l 

CHORDATA 142 



134 
134 

135 



137 
137 



XIV • CONTENTS 



chapter 14 Evolution in Plants 144 

CYANOPHYTA 146 

RHODOPHYTA 146 

PYRROPHYTA AND CHRYSOPHYTA 146 

PHAEOPHYTA 148 

EUGLENOPHYTA AND CHLOROPHYTA 148 

SCHIZOMYCOPHYTA 148 

MYXOMYCOPHYTA AND EUMYCOPHYTA 149 

OVERLAPPING SYSTEMS OF 
CLASSIFICATION 149 

BRYOPHYTA 150 

TRACHEOPHYTA 151 

ORIGIN OF VASCULAR PLANTS 151 

chapter 15 Genetic Evidence 155 

HYBRIDIZATION 155 

DOMESTICATED SPECIES 158 

GENE AND CHROMOSOME HOMOLOGY 159 

1 *"" THE HEREDITARY MATERIAL 160 

PART III 

The Mechanism of Evolution 

chapter 16 Mendel's Laws 166 

^ SEGREGATION 168 

INDEPENDENT ASSORTMENT 173 

chapter 17 Variation Due to Recombination . . 177 

MULTIPLE ALLELES 177 

BACKGROUND EFFECTS 180 

RECOMBINATION AND INTERACTION 180 

chapter is The Physical Basis of Evolution ... 185 

' MITOSIS 185 

LIFE CYCLE IN ANIMALS 188 

LIFE CYCLE IN PLANTS 189 

MEIOSIS 190 

SEX DETERMINATION 191 

SEX LINKAGE 191 



CONTENTS • XV 



chapter 19 Linkage 195 

' LINKAGE AND CROSSING OVER 195 

" LINEAR ORDER OF THE GENES 196 

chapter 20 Chromosomal Variation 199 

"DUPLICATION AND DEFICIENCY 199 

INVERSION 201 

* TRANSLOCATION 202 
» POSITION EFFECT AND 

PSEUDOALLELISM 203 

HETEROPLOIDY 204 

' POLYPLOIDY 204 

chapter 21 Mutation 207 

■ TYPES OF MUTATIONS 207 

INDUCED MUTATION 209 

MUTATION RATES 210 

CONTROLLED GENETIC CHANGES 212 

THE MUTATION THEORY OF DE VRIES 213 

chapter 22 Quantitative Inheritance 216 

• GENETICS OF QUANTITATIVE TRAITS 218 
, HETEROSIS 220 

chapter 23 Variation in Natural Populations . . 225 

GENETIC ANALYSIS OF NATURAL 
POPULATIONS 226 

V CHROMOSOMAL VARIATION 229 

chapter 24 Genetics of Populations 234 

THE HARDY-WEINBERG EQUILIBRIUM 235 

' MUTATION 237 

chapter 25 Natural Selection 239 

ARTIFICIAL SELECTION 24l 

SELECTION FOR RESISTANCE 242 

THE BALDWIN EFFECT 244 

THE THEORY OF SELECTION 245 

SELECTION AND MUTATION 247 



XVI • CONTENTS 



chapter 26 Polymorphism 249 

TRANSIENT POLYMORPHISM 250 

^THE ORIGIN OF DOMINANCE 252 

BALANCED POLYMORPHISM 254 

HETEROSIS AND POLYMORPHISM 256 

chapter 27 Genetic Drift 262 

EFFECTIVE SIZE OF POPULATIONS 264 

GENETIC DRIFT AND EVOLUTION 266 

chapter 28 The Origin of Subspecies 268 

POPULATION STRUCTURE 268 

ISOLATION AND SUBSPECIATION 271 

GENETIC DIFFERENCES BETWEEN 
SUBSPECIES 272 

chapter 29 Hybridization and Evolution 277 

THE EFFECTS OF MIGRATION 278 

INTROGRESSIVE HYBRIDIZATION 280 

POLYPLOIDY AND EVOLUTION 281 

chapter 30 Isolating Mechanisms 286 

TYPES OF ISOLATING MECHANISMS 286 

THE ORIGIN OF ISOLATING 
MECHANISMS 289 

chapter 31 The Origin of Species 291 

THE SPECIES AS A BIOLOGICAL UNIT 292 

MODES OF EVOLUTION 293 

THE ORIGIN OF HIGHER TAXONOMIC 
GROUPS 296 

CHAPTER 32 Evolution of Genetic Systems 301 

GENETIC RECOMBINATION 301 

^ASEXUAL VERSUS SEXUAL 
REPRODUCTION 303 

^ HAPLOIDY VERSUS DIPLOIDY 304 

^THE SEPARATION OF THE SEXES 306 

V SEX DETERMINATION 306 

SEXUAL DIFFERENTIATION 310 

THE CONTROL OF RECOMBINATION 313 

SEXUAL SELECTION 315 



CONTENTS • XV11 



PART IV 
Evolution and Man 



chapter 33 Human Evolution 

THE PROSIMIANS 323 

THE HIGHER PRIMATES 325 

FOSSIL PRIMATES 330 

THE FOSSIL RECORD OF MAN 
THE ORIGIN OF MODERN MAN 
MAN, A POLYTYPIC SPECIES 
THE RACES OF MAN 343 

CULTURAL EVOLUTION 345 



323 



332 

338 
341 



chapter 34 Radiation, Genetics, and Man .... 350 

THE FREQUENCY OF HARMFUL GENES 350 

GENETIC EFFECTS OF RADIATION 353 

SOMATIC EFFECTS OF RADIATION 355 

RADIATION EFFECTS IN MAN 356 

chapter 35 Man as a Dominant Species 360 

ELEMENTARY DEMOGRAPHY 363 

THE CAUSES OF OVERPOPULATION 366 

THE REGULATION OF MAN'S 

INCREASING NUMBERS 368 



chapter 36 Man's Future 373 

man's future as a species 373 
man's future numbers 374 
man's genetic future 375 
eugenics 376 

Appendix 379 

A: from Charles darwin's Voyage of the 
Beagle 381 

B: from thomas malthus' Essay on the Principle 
of Population 389 

Glossary 399 

Index 409 




PART 



Introduction 



CHAPTER 



1 



Adaptation 



In this world are many strange and wondrous sights, 
but the one that most easily arouses a sense of the ludicrous 
nature of things is the slightly balding, slightly paunchy, slightly 
middle-aged father bouncing on his knee a baldish, pot-bellied 
infant, a replica of himself not only in general but in many par- 
ticulars. This is the joke he has played on encroaching old age, 
and around the process by which it has come to pass has always 
hung an aura of mystery, myth, taboo, superstition, and mirth. 
Despite the intense interest man has always shown in his own 
self -duplication, only in the last century has any real progress 
been made toward an understanding of the process. The sight of 
doting parents and their offspring raises still broader questions, 
however. How far back into the mists of antiquity does this living 
chain extend? What was its beginning? And how far into the 
future will it persist ? Here, too, knowledge has accumulated at an 
accelerating pace during the past century. In many ways, our 
knowledge and understanding of heredity and evolution have 
developed hand in hand, for the physical basis of heredity is also 
the physical basis of evolution. But man is only one species. He 
lives on a ball of matter spinning in space and populated by bil- 
lions of individuals belonging to millions of different species, as 
diverse in nature as bacteria and orchids, honey bees and humans. 
This situation seems very improbable, for a living organism ap- 
pears to contradict, even to defy, the ordinary laws of chemistry, 
physics, and thermodynamics. The question is, What is the origin, 
the history, and the future of this great variety of individualized 



3 



4 • INTRODUCTION 

protoplasm? We cannot hope at present to know all of the answers, but our 
knowledge has increased to the point where we now know something of what 
has happened in the past and of the mechanisms responsible for the changes that 
have occurred. 

The physical evidence for the study of this question consists of the 
species of animals and plants now living and of the fossils, which are the rem- 
nants or traces of organisms that have lived in the past. For the moment, let us 
consider the living species. One feature common to the great variety of living 
things is that they are adapted for life in the environment in which they are 
found. Obviously, if they were not adapted to their environment, they would not 
be found there; they simply could not survive. However, each species is adapted 
to a somewhat different set of environmental conditions from every other species. 
Not only are fish found in water, monkeys in trees, and antelope on the prairie, 
but each different species of fish tends to have its own habitat, as any good fisher- 
man (or ichthyologist, for that matter) will testify. Adaptation is so universal 
and so self-evident that we tend to overlook or to ignore it, but it is a basic bio- 
logical fact. Each living organism has a particular set of adaptations peculiarly 
suited to its mode of life. In fact, the adaptations are so precise in so many cases 
that they appear exactly suited to the needs of the organism in its environment. 
A fish, for example, in order to move about in the water in which it lives, obvi- 
ously needs appendages such as the fins. To speak of the "needs" of the organ- 
ism, however, is to run the risk of being teleological. Such usage, which often is 
a reflection of a way of thinking, has considerably hampered the study of adapta- 
tion. Just because an organism is constructed in a certain way or behaves in a 
certain way is no indication that it necessarily has any recognition of its needs or 
that any conscious purpose or plan governs it. On the other hand, lack of recog- 
nition of its needs by the organism does not indicate a lack of functional signifi- 
cance in its structure or behavior. A fin is for swimming, and a wing for flying, 
entirely aside from the question of needs or cognition. 

Types of Adaptation 

Two general types of adaptation may be distinguished. One type might 
be called individual adaptation, by which an organism, through suitable modi- 
fications in its physiology, adjusts to environmental stresses. Fair-skinned people, 
for example, when exposed to sunlight, typically become "tanned." Even though 
this change is an individual response to a particular stimulus, it is ultimately 
under the control of that person's hereditary make-up or genotype, for not all 
people have the ability to form melanin in response to exposure to sunlight. 
Albinos and people with very light complexions may continue to sunburn despite 
continued exposure to the sun; the ability to tan is simply beyond the capacity of 
their genotypes. The discomfort of such people could be considered sufficient evi- 



ADAPTATION • 5 

dence of the adaptive value of the ability to tan, but it would be desirable to 
know more about the process. On the other hand, some human populations are 
much more heavily pigmented than others, the pigment developing even though 
the individuals may not be exposed to the sun. In the dark-skinned races, pig- 
ment is formed under the control of the genotype also, but no external stimulus 
is needed. In these races, population adaptation may be said to exist, for the 
whole population routinely has darkly pigmented skin. There seems little reason 
to doubt that the skin pigment of the dark-skinned races has adaptive value just 
as it does in the case of individual adaptation, but the exact nature of this adap- 
tive value at present remains a matter of speculation. The two types of adapta- 
tion, individual and population, are rather different although both are under 
hereditary control. One of the more intriguing questions in evolutionary research 
is how individual adaptation may be transformed into population adaptation. It 
may seem to verge on the question of the inheritance of acquired characteristics 
but is nonetheless quite distinct from it. 

Although each species is unique in its adaptations to its own particular 
physical and biological environment, nevertheless all species face essentially the 
same basic problems. The variety of different kinds of adaptations represent dif- 
ferent solutions to these problems. For example, oxygen is required in the 
metabolism of fish and mammals (and most other species) ; the fish extract 
oxygen from the water through their gills, but the mammals use quite different 
structures — the lungs — to obtain oxygen from air. The basic problems confront- 
ing every species, if it is to continue to exist, are very simple: it must survive, 
and it must reproduce. In order to survive, an organism must obtain an adequate 
supply of food; it must have some measure of protection from other organisms, 
whether predators, competitors, or parasites; and it must make suitable adjust- 
ments to the existing physical conditions. Survival alone is not enough, however. 
If, at a given time, all the members of one species survived through maturity to 
old age without reproducing, that species would become extinct with that 
generation. 

No adaptation is perfect. With the variety of functions required of the 
organism, the adaptations achieved must be, perforce, a compromise among all 
these functions. The organism is a complex bundle of adjustments to its neigh- 
bors of all degree and to its physical environment. 

The Environment 

The nature of the environment is worthy of comment, for it will em- 
phasize the variety of adaptations required for survival and reproduction. The 
physical environment consists of some sort of substrate; this may be fresh or salt 
water, or land, or air, or, for the parasites, another organism. Fresh water alone 
represents a variety of substrates requiring somewhat different adaptations for 



6 • INTRODUCTION 

survival — in lakes, rivers, streams, ponds, swamps, and so on — whereas each 
different species represents a different substrate for the parasites. Another limit- 
ing physical factor is temperature. Different species may have somewhat different 
ranges of temperature tolerance, but the actual range at which any life as we 
know it is possible is really rather narrow. Strangely enough, this range happens 
to coincide with existing temperatures on the earth. Other forces such as pressure 
and gravity are a constant part of the environment. Furthermore, sound waves, 
light waves, and chemical particles are constantly impinging upon the organism. 
The biotic environment of an organism consists, first, of other members 
of the same species, which interact with each other in various ways. In relation 
to reproduction there may be courtship and care of the young. There may also be 
various group activities — colony formation or migration, for example — requiring 
some degree of cooperation. Competition between members of the same species 
may develop in the quest for food or in the establishment of nesting territories. 
Many adaptations appear to be related to these functions. Furthermore, the rela- 
tions between different species may be as diverse as predation, parasitism, compe- 
tition, and cooperation. 

Adaptation in the Frog 

Thus far, our discussion has been rather general, and it may be helpful 
to consider briefly the problems of adaptation as they have been solved by one 
species. The leopard frog, Ran a pipiens, has been widely used in zoological 
laboratories in the United States. Because it is so familiar, the frog is especially 
suitable for reappraisal here in terms of its adaptations rather than of its organ 
systems. In so doing, we may seem to belabor the obvious. 

To survive, the frog is confronted with the problem of finding and 
securing an adequate supply of food. To move about in this search, the frog has 
legs, which are adapted for swimming in water and for jumping on land. The 
webbed feet are obvious adaptations for swimming. However, since the legs 
function for locomotion in or on two media, they represent an adaptive com- 
promise, and it is quite clear that the frog is not very efficient at moving about 
in either. His search for food is guided by the major sense organs of sight, hear- 
ing, smell, and taste, which serve as receptors of more or less distant stimuli. It 
is a rather remarkable fact, though you may not at first so consider it, that all of 
these major sense organs are localized in the head, which is at the front end of 
his bilaterally symmetrical body. (Bilateral symmetry — that is, an arrangement of 
the body into anterior and posterior ends, and dorsal and ventral surfaces — is an 
adaptation to an active life. Sessile species are generally radially symmetrical; 
that is, their body parts are arranged about a central axis.) It would seem quite 
a coincidence that these sense organs are so strategically placed at the anterior 
end, which is constantly probing into new parts of the environment. Imagine 



ADAPTATION • 7 

how much less useful these structures would be if arranged on the frog's 
posterior. 

Once the food has been located, the mouth assumes the problem of 
securing it. The tongue, unlike man's, is attached at the front of the mouth 
cavity and is flicked out with speed and precision to pick off unwary insects that 
come within reach. The vomerine teeth, in the roof of the mouth, crush the 
insects before they pass into the digestive tract. In the digestive system, the food 
is broken down into molecules that can be absorbed through the walls of the 
intestine and transported by the circulatory system to the immediate vicinity of 
the individual living cells. The respiratory system is also tied in with the circu- 
latory system so that the oxygen essential for the utilization of the food mole- 
cules during the metabolic activity of the cells is made available to them. The 
waste products of cellular metabolism are in turn removed by the circulatory 
system, carbon dioxide (C0 2 ) being eliminated primarily from the lungs and 
nitrogenous wastes by the kidneys. The frog's digestive system, respiratory sys- 
tem, circulatory system, and excretory system are fundamental adaptations for 
supplying the necessary metabolic raw materials to the living cells and removing 
the waste products after the cells have extracted energy and essential compounds 
from them. Without adaotations of this sort, multicellular life would not be at 
all possible. 

Furthermore, the organism acts as an integrated whole, not merely as a 
collection of cells, tissues, and organs. This integration is due to chemical co- 
ordinating systems, mainly hormonal, and to the nervous system. As a result, the 
individual cells become interacting and interdependent parts of a well-integrated 
unit. These chemical and nervous mechanisms operate in such a way that even 
under stress a balanced internal environment is maintained. Maintenance of an 
internal dynamic equilibrium is called homeostasis. 

There are several ways in which the frog secures some degree of protec- 
tion from other organisms. The sense organs and the locomotor system obviously 
serve a dual purpose, in securing food and escaping predators. The dorsal place- 
ment of the eyes and nostrils is adaptive in that the frog can remain almost 
completely submerged in water, and yet it can breathe and see above the surface. 
Placement of the eyes in the skull is an adaptive feature, as can be easily ob- 
served by comparing the angles of vision in a carnivore like the cat and an 
herbivore such as the rabbit. 

The coloration of the leopard frog has considerable protective value. 
The basic color is a cryptic green or greenish brown, matching the tall grass or 
weeded bank that is the frequent habitat of this species. By its ability to regulate 
the degree of dispersion of the pigment granules in its chromatophores, the frog 
is capable of considerable change in shade to match its background. Moreover, 
the outline of the body is broken up by the numerous spots on the skin. This so- 
called disruptive pattern destroys the visual impression that would otherwise be 



8 • INTRODUCTION 

gained of the frog's size and shape, and it is especially effective when observed 
(or not observed) in the pattern of light and shadow created in a grassy meadow 
on a sunny morning. Even to details, the disruptive effect is much in evidence; 
the eye is masked to some extent by a dark line that seems to run through it, and 
the matching up of the spots on the upper and lower parts of the hind legs 
creates a series of dark bands running at right angles to the length of the long 
bones, disrupting the outline of these otherwise quite prominent appendages. It 
should be noted that all of this coloration is found only on the dorsal surfaces of 
the body; the ventral surfaces are creamy white. This pattern of dark above and 
light below is known as countershading, and its adaptive significance lies in the 
fact that the frog when seen from below in the water will be very light, match- 
ing the sky. (For a most interesting and authoritative account on the functional 
significance of animal coloration, see Cott's Adaptive Coloration in Animals.) 
In addition to its concealing function, the skin serves as a more or less effective 
barrier to infection by a variety of parasites and as a respiratory organ. 

The frog is a rather stupid animal with quite stereotyped behavior. It 
escapes the notice of its predators by remaining motionless; if alarmed suffi- 
ciently, it gives a series of explosive leaps and then once again freezes. If it 
jumps into the water, it burrows into the mud or debris for concealment. These 
behavior patterns, though simple, are clearly adaptive for the protection of the 
frog from predators. However, leopard frogs appear to have a rather complex 
pattern of migratory behavior. In the spring they migrate to the breeding ponds, 
and then, after breeding, apparently move on to summer feeding territories. In 
the fall, as colder weather ensues, large-scale migrations to over-wintering sites in 
lakes and streams take place. These migratory patterns are clearly adaptive. 

In winter, the air temperature drops below the range at which the frogs 
can remain active, and to survive, they burrow into the debris at the bottom of 
ponds and streams. Other controlling physical factors in the life of the frog 
include moisture. Though leopard frogs seem less closely tied to damp areas than 
most other amphibian species, it is clear that this species too may be subject to 
dehydration rather quickly. Certainly they are much less in evidence in open 
meadows on sunny, dry, and windy days than on cloudy and humid days. 

Perhaps the most remarkable adaptations of all are those related to 
reproduction. Reproduction in the leopard frog occurs in the spring in rather 
shallow pools. The calling males congregate in large numbers at the breeding 
site. The females are attracted to the site, deposit their eggs while clasped in 
amplexus by the males, and depart. The physical and biological factors that 
initiate and control this elaborate series of events are in most instances matters 
of conjecture; for example, we do not know what determines the selection of 
the breeding site, which must not dry up before the tadpoles metamorphose. The 
obvious differences between males and females are not great, the nuptial pads 
and the song of the males during the breeding season being the most noticeable. 



ADAPTATION • 9 




Fig. 1-1. Life cycle of the leopard frog, Rana pipiens. 



At every stage in the life cycle (Fig. 1-1), adaptations appear. The egg mass of 
pipiens from the warm southern parts of the United States is rather flattened, 
whereas that of females from the northern states is globular; there is an obvious 
relation to the lower oxygen concentration in warm water as compared to cold. 
The eggs are countershaded. The larva that emerges is an aquatic animal, swim- 
ming with fins and respiring with gills. Unlike the adult, it is an herbivore, with 
its digestive tract correspondingly adapted for handling this different type of 
food. The remarkable series of changes known as metamorphosis then occurs, 
with the adult frog, a terrestrial tetrapod, the result. 



10 • INTRODUCTION 

These, then, are some of the adaptations in the frog; for the most part 
they are not particularly striking or unusual. The frog was chosen as an example 
to illustrate the fact that even the most familiar species is quite precisely adapted 
to its ecological niche. As species go, the leopard frog must be regarded as sort 
of a fringe dweller, firmly established neither in water nor on land. Yet in this 
marginar^rrvkonment, which is its normal habitat, the frog has been quite suc- 
cessful by the only* criterion we have for measuring biological success — that is, 
survival as a species. Mere survival may not seem at first glance to be a very lofty 
criterion by which to judge success, but at least it is objective. Certainly this 
evolutionary line has outlasted some more impressive and dominant species that 
have lived in the past, such as the mammoth, the saber-toothed tiger, and all of 
the dinosaurs. 

Protective Coloration 

The discussion of adaptation sometimes tends to dwell on the more 
spectacular types of adaptive changes, some of which — among them, protective 
coloration — are extremely fascinating. The adaptive value of animal colors has 
sometimes been doubted. For example, when the Nile catfish was found to show 
reversed countershading — that is, the dorsal surface light and the ventral surface 
dark — the whole theory of countershading was brought under suspicion. How- 
ever, the concept was doubly strengthened when it was discovered that this fish 
characteristically swims upside down. 

Not only are colors frequently adapted for concealment of the organ- 
ism, but the animal may enhance the protective value of its coloration by its 
behavior. Certain moths are cryptically colored to match the bark on which they 
ordinarily rest, and in addition they hold their wings flat against the bark, which 
eliminates the shadow, and position their bodies in such a manner that their 
pattern best matches the pattern of the bark (Fig. 1-2). 

The coloration of some animals is adapted not so much for concealment 
by blending in with the background of its habitat as it is for disguise, by which 
they resemble some other object in their environment. A number of species — for 
example, butterflies and other insects, fish, and frogs — resemble leaves; still 
others resemble twigs or lichens. A most peculiar group are the geometrid moths 
that resemble bird droppings, especially startling when they fly away. There is 
much in common between the desert lizard, which lures unwary, insects to their 
deaths because the corner of its mouth when opened resembles a small red desert 
flower, and the anglerfish, which has a dorsal spine modified into a lure that 
dangles before its gaping mouth. 

In some species the so-called aposematic colors serve as advertisements 
rather than as disguise or concealment. The skunk, with his striking black and 
white colors, is not easily missed nor is he easily mistaken for any other species. 
The white flag of the Virginia white-tailed deer appears to serve as a warning 



ADAPTATION • 11 




Fig. 1-2. Willow beauty moth (Boarmia gemmaria) resting on bark. Con- 
cealment is achieved by the similarity between the wing pattern and the 
bark, and is further enhanced by the horizontal positioning of the body and 
the elimination of shadows from the wings. (Courtesy of Cott.) 



signal. In birds, the same colors used by the male as a part of the courtship dis- 
play may also be used in a threat display toward other males invading his 
territory. 

The insects with stings, such as bees, wasps, and hornets, are usually 
strikingly colored black and yellow and tend to some extent to resemble each 
other. This type of mimicry, in which a number of dangerous or unpalatable 
species resemble one another, is known as Mullerian mimicry. This is distinct 



12 • INTRODUCTION 

from Batesian mimicry, in which the harmless species resemble the harmful or 
nauseous types. A classical example of mimicry is the resemblance of the Viceroy 
butterfly {LlmenUis archippus) to the Monarch (Danaus plexippus). The 
Viceroy is colored orange and black like the Monarch and is quite different from 
the other members of its own genus, which are black with white spots. Originally 
thought to be a case of Batesian mimicry, this example may not fit either classical 
pattern, for recent evidence has shown that the Viceroy, though more palatable 
to birds than the Monarch, is eaten somewhat less often than other butterflies. 
The whole subject of mimicry is of extreme interest, and much work remains to 
be done to clarify many of the questions in this field. 

Examples of remarkable adaptations could be cited almost endlessly, 
but only one more will be mentioned. A certain shrike in Ceylon (Hemipus 
picatus) builds its nest on the bare limbs of trees. The nest is so constructed that 
it resembles a knot, and is cunningly camouflaged with bits of bark and lichen 
to heighten the effect. The young birds are cryptically colored so that they blend 
with the nest. Most remarkable of all, however, is the fact that the birds sit 
facing each other with their eyes partially closed and their beaks pointing up- 
ward and almost touching. The total effect of the cooperative efforts of parents 
and young is that of a knot on a dead branch with just a small stub of a broken 
branch protruding from the knot. To visualize how these complex behavior pat- 
terns became incorporated into the hereditary make-up of this species, as they 
clearly must be, is to stretch the imagination. 

Adaptation in Man 

Although we tend to think of man as having mastered his environment, 
actually he is adapted to rather specific environmental conditions, and his mastery 
is due to his skill in modifying the environment to approximate his needs rather 
than in broadening his environmental tolerances. Man is a terrestrial animal, and 
was undoubtedly confined to the tropics and subtropics until his relatively recent 
discovery of the use of fire and clothing. His erect bipedal locomotion is adapted 
to life in relatively open country rather than to heavily forested or mountainous 
regions. His lungs enable him to extract oxygen from the air, and he requires 
an adequate daily supply of fresh drinking water. Though an omnivore, he is 
ultimately dependent on green plants for all of his food. This analysis could be 
extended, but it should suffice to demonstrate that man, too, makes well-defined 
demands on his environment. 

SUGGESTED READING 

Carpenter, G. D. H., and E. B. Ford, 1933. Mimicry. London: Methuen. 
Caspari, E., 1951. "On the biological basis of adaptedness," Am. Scientist, 39:441- 
451. 



ADAPTATION • 13 

Cott, H. B., 1940. Adaptive coloration in animals. New York: Oxford University 
Press. 

Emerson, A. E., I960. "The evolution of adaptation in population systems," Evolu- 
tion after Darwin, Vol. I, The evolution of life, S. Tax, ed. Chicago: Uni- 
versity of Chicago Press. 

Huxley, J. S., 1943. Evolution. The modern synthesis. New York: Harper. 

Muller, H. J., 1950. "Evidence of the precision of genetic adaptation," Harvey Lec- 
tures, 43:165-229. 

Portmann, A., 1959. Animal camouflage, A. J. Pomerans, tr. Ann Arbor: University 
of Michigan Press. 

Simpson, G. G., 1953. The major features of evolution. New York: Columbia Uni- 
versity Press. 

Stephenson, E. M., and C. Stewart, 1955. Animal camouflage, 2d ed. London: Black. 

Waddington, C. H., I960. "Evolutionary adaptation," Evolution after Darwin, 
Vol. I, The evolution of life, S. Tax, ed. Chicago: University of Chicago 
Press. 



CHAPTER 



Evolutionary Thought 
before Darwin 



Although thought on the origin of species has apparently 
preoccupied men of almost every culture, much of the speculation 
has been of such a nature that it must be regarded as based largely 
on myth, superstition, or vague philosophical ideas rather than on 
careful observation and the accumulation of facts. Furthermore, 
the modern reader may read into the statements of earlier writers 
things they did not intend to say. In this short review we obvi- 
ously cannot hope to trace the complete history of the develop- 
ment of the evolution concept. Instead a sampling of the ideas 
advanced at different periods will be presented in an effort to 
convey some of the flavor of the thinking of different ages. 

Greek Thought 

Among the Greeks, Anaximander, who lived in the 
sixth century B.C. (611-547 B.C.) merits attention, for he at- 
tempted to explain the origin of the universe on a rational basis 
rather than by myths or legends. He visualized all things as hav- 
ing come from a primordial fluid or slime to which they ulti- 
mately return. Living things, both plant and animal, were formed 
as this mud dried. This concept appears to be one of the earliest 
known theories of spontaneous generation. Man himself was first 
shaped like a fish and lived in the water. Later, when he became 
capable of terrestrial life, he burst forth from his fishlike capsule 

14 



EVOLUTIONARY THOUGHT BEFORE DARWIN • 15 

like a butterfly from its chrysalis and assumed human form and a life on land. 
This theory was crude, yet the implication of evolution is clear. 

Xenophanes (576P-480 B.C.), believed to have been a pupil of Anaxi- 
mander, is the first person known to have recognized that fossils were the rem- 
nants of once-living organisms and that marine fossils on land indicated that the 
sea formerly covered the earth. 

Empedocles in the fifth century B.C. (495-435 B.C.) stated that the four 
elements were air, earth, fire, and water, and that these elements were acted upon 
by two forces, love and hate, which caused their union or separation. He also 
suggested that plants had arisen first, and that animals were later formed from 
them. The germ of the idea of natural selection was contained in his belief that 
the parts of animals were formed separately and then united at random by the 
triumph of love over hate. Most would then be monsters and unviable, but a 
few could survive. He and many others, both before him and for centuries after- 
ward, believed in the possibility of spontaneous generation of life from nonliving 
materials, and thus settled, in rather simple fashion, the question of the origin of 
life. 

Aristotle (384-322 B.C.), whose ideas dominated biological thought for 
well over a thousand years, was the greatest of the Greek men of science. He 
was a vitalist, believing that living things were animated by a vital force or 
guiding intelligence quite different from anything to be found in nonliving 
matter. In this idea he was preceded by Anaxagoras (500-428 B.C.), but to 
Aristotle this internal force became a perfecting principle, operating constantly 
to improve or perfect the living world. Growing out of this concept was his 
ladder of nature ("Scala naturae") or chain of being in which he arranged living 
things on a scale of perfection. The succession ranged from inanimate matter 
through the lower plants to the higher animals on a single scale with man, at the 
top, being the most nearly perfect. Aristotle apparently never interpreted the 
chain as possibly suggesting that each group had evolved from the one below it. 
He believed in spontaneous generation not only for smaller animals but for 
larger ones such as frogs and snakes. He thought that the inheritance of mutila- 
tions was rather common, but rejected the idea of the inherited effects of use and 
disuse. Adaptation to him was the result neither of the survival of accidental 
fitness, as it was for Empedocles, nor of functional modifications but rather of 
the action of the perfecting principle. Thus Aristotle did not add in any direct 
way to the development of modern evolutionary thought despite his many con- 
tributions to biology. Since he remained the most authoritative source of biolog- 
ical information for so long a period, it could be argued that some of his theories 
actually hampered the development of the theory of evolution. The difficulties, 
however, lay less with Aristotle than with the nature of the times that followed 
him. 

For centuries after Aristotle little progress was made toward a better 



16 • INTRODUCTION 

understanding of evolution, for the spirit of inquiry that characterized the Greeks 
gradually withered away and died. Epicurus (341-270 B.C.) is worth mention- 
ing, not because he added significantly to evolutionary thought but because he 
attempted to explain the world and the universe as natural phenomena governed 
by natural causes. As a materialist or mechanist, he tried to combat the super- 
stitious beliefs in supernatural forces ruling the universe. In this effort he op- 
posed the Aristotelian argument of teleology, or the grand design or purposeful- 
ness of events, which was widely accepted at the time. As a part of his philos- 
ophy he adopted the atomic theory of Democritus (460P-362? B.C.). 

The Decline of Science 

The Roman poet Lucretius (99-55 B.C.) was a follower of Epicurus, 
and in his famous work, On the Nature of Things (De Rerum Natura), summed 
up most of the Greek non-Aristotelian thought. Lucretius is significant, not for 
any particular advance in evolutionary thought, but because he marked the end 
of a period of thought, and through his work preserved the atomic theory during 
the Dark and Middle Ages and gave a forceful restatement of the mechanistic 
position. In his rejection of Aristotle's teleology, he also rejected much of the 
rest of Aristotle's work, and thus did not achieve a complete synthesis of the 
best of Greek thought. 

The Roman Pliny (a.d. 23-79) compiled a tremendous store of infor- 
mation and misinformation in his Natural History, which served as man's 
primary source of knowledge about natural history for nearly 1500 years. He 
was not primarily an investigator, however, and his uncritical recitation of the 
work of others added nothing new. Galen (a.d. 130-200), the last important 
biologist of antiquity and the personal physician of Marcus Aurelius, made in- 
vestigations in anatomy and physiology that were accepted as authoritative for 
centuries, but he, too, made no direct contribution to evolutionary theory. Thus, 
at the close of the classical period some few ideas that had a bearing on evolution 
had been expressed, but the concept was far from its modern form. 

Although the decline of ancient science has at times been attributed to 
the rise of Christianity, this seems hardly to have been the case. The decline set 
in long before the birth of Christ and even at the time of Galen's death, in 
a.d. 200, the Christians were only a small group without influence. Preoccupa- 
tion with spiritual matters did little to advance science, and active conflicts did 
develop later, but no one church can claim any monopoly on this sort of opposi- 
tion. For centuries the churches were the primary centers of learning. Such lead- 
ers among the early Christians as St. Augustine (354-430) and much later 
St. Thomas Aquinas (1225-1275), who has remained an authority of the 
Church, rejected a literal interpretation of the story of special creation in Genesis 
and suggested instead an allegorical naturalistic interpretation patterned after 
Aristotle. However, throughout the Dark Ages no progress was made in the 



EVOLUTIONARY THOUGHT BEFORE DARWIN • 17 

development of the theory of evolution. The rise of Scholasticism in the thir- 
teenth century led to the study of the writings of the ancients on nature but to 
little study of nature itself. Much of this material was obtained from translations 
of works in Arabic, many of which had in turn been derived from the Greek. In 
the reaction by the Church in 1209 against Arabian science and philosophy, the 
study of Aristotle was also banned, but this interdiction was later relaxed. This 
period marked the beginning of the trend toward a literal interpretation of the 
seven days of creation, a trend that predominated for centuries. The Spanish 
Jesuit Suarez (1548-1617) was among those who argued strongly in favor of a 
literal interpretation of Genesis and refuted Augustine and Thomas Aquinas. 
The result was that for three centuries, from the sixteenth to the middle of the 
nineteenth, Special Creation was official Church doctrine even though it was a 
departure from the beliefs of some of the earlier leaders of Christianity. Diver- 
sity of opinion was denounced as heresy, and free discussion of the concept of 
evolution carried with it the risk of reprimand or excommunication by the 
Church even up to the time of Buffon in the late eighteenth century. Whether 
this attitude aided or hindered the development of the theory of evolution is 
hard to say, but it did play a significant part in the history of the concept. 

The Renaissance 

The revival of the classical art and learning of the Greeks and Romans, 
which was known as the Renaissance, took place during the fourteenth, fifteenth, 
and sixteenth centuries. This development, in turn, led to a rebirth in the spirit 
of inquiry; the Renaissance was not, however, marked by any notable progress 
on the question of the origin of species. Leonardo da Vinci (1452-1519) real- 
ized that the fossil marine shells that he found in the Apennine mountains indi- 
cated that they must once have been covered by the sea, but he did not develop 
the idea in relation to biological evolution. Similarly, Cesalpino (1519-1603) 
suggested that flower petals were modified leaves, another concept that could 
have led to the theory of evolution. Most of the naturalists of the time were 
Encyclopedists who made every effort to collect all the known facts about living 
things. The discovery by Harvey (1578-1657) of the circulation of the blood in 
a sense marks the transition from the biology of the ancients to modern experi- 
mental biology. 

The Natural Philosophers 

In the seventeenth and eighteenth centuries a number of men now 
known as the natural philosophers tried to develop unified systems of thought 
by which they could interpret the universe. Since life is a part of the universe, 
biological matters were included in their schemes of things. Although their inter- 
ests were not always primarily biological, they did make some advances in evolu- 
tionary thought. We will mention here just some of the biological insights of a 



18 • INTRODUCTION 

few of these men. Francis Bacon (1561-1626) called upon men to seek knowl- 
edge by observation, experiment, and inductive reasoning, and to free themselves 
from both Scholasticism and Aristotelean philosophy. He strongly urged that 
the variations in nature should be studied and their causes determined. Further- 
more, he pointed out that artificial selection among these variations could be 
used to cause species to change and that transitional forms exist in nature. Al- 
though his examples were somewhat farfetched — he suggested, for example, that 
flying fishes were intermediate between fishes and birds, and bats between birds 
and quadrupeds — the fact remains that even at the opening of the seventeenth 
century the question of the fixity of species was being raised. 

Bacon proposed methods by which the nature of the universe could be 
determined, but Descartes (1596-1650) was the pioneer among the systematic 
philosophers who speculated on the nature of the system itself. Guarded in his 
expression, he postulated that the universe could be explained on physical prin- 
ciples. This mechanistic approach had a great impact on biology, especially since 
it came just after Harvey's success in explaining the circulation of the blood in 
physical terms. Descartes was circumspect in presenting his ideas out of fear of 
offending the Church, and his writings on physiology, which became the founda- 
tions of modern physiology, were withheld from publication until after his death. 
Since he spoke in terms of the evolution of the universe, and life was a part of 
this system, the evolution of life was more or less indirectly included. 

Leibnitz (1646-1716) had a better scientific background than his 
predecessors, for he understood the nature and origin of fossils, had extensive 
knowledge of plant and animal classification and of comparative anatomy, and 
was familiar with the wonders revealed by the recently discovered microscope. 
His doctrine of continuity applied to life was still another revival of the Aris- 
totelean chain of being, but it did not necessarily lead him to the concept of evo- 
lution. He did, however, speculate on the relationship between the fossil am- 
monites and the living nautilus and even suggested that major changes of habitat 
might cause changes in animal species. He stated that his doctrine of continuity 
led to the idea that intermediate species should exist, but he shied away from 
the thought of species intermediate between man and the apes, saying that if 
they existed, it must be in another world. Kant (1724-1804), who has often 
been cited as a predecessor of Darwin, was undoubtedly familiar with the sug- 
gestion that species change but he apparently never embraced the idea of evolu- 
tion completely. 

Biological Research and Writings 

Just as the natural philosophers influenced the thought and direction of 
research of the biologists of their day, they, in turn, were influenced by the ad- 
vances being made. One such advance was the development of a system of classi- 



EVOLUTIONARY THOUGHT BEFORE DARWIN • 19 

fication for plants and animals. The foremost predecessor of Linnaeus (1707- 
1778), who is universally regarded as the father of the modern binomial system 
of nomenclature, was John Ray (1627-1705), an English naturalist. Ray wrote a 
number of systematic works, primarily on plants but also on animals, that repre- 
sented major advances toward the "natural system" of classification, which takes 
into account all known similarities and differences. It was Ray who first clearly 
defined the species concept as being related to community of descent and inter- 
fertility rather than to fixity of type, but he did not extend this idea in the 
direction of evolution. Linnaeus himself in the tenth edition (1758) of his 
Systema Naturae established the foundation on which taxonomy has since been 
built. His scheme was a branching one, rather than a chain or ladder form, and 
living things were named according to genus and species — man, for example, 
being Homo sapiens. Althought he developed a branching system, Linnaeus at 
first believed in the fixity of species; as his experience broadened, however, he 
came in later editions to accept the possibility of evolution, at least within the 
genus, due either to hybridization or the effects of environment. 

The work of de Maupertius (1698-1759) has recently been rescued 
from an undeserved obscurity. Eminent in his own day, he aroused the wrath of 
Voltaire, whose bitter mockery has undoubtedly colored the opinions of posterity. 
His arguments against the preformation doctrine in embryology preceded those 
of Wolff by fifteen years. Moreover, he developed a particulate theory of heredity 
based on experiments in animal breeding and investigations of human heredity, 
applying probability theory to his findings a century before Mendel. In addition 
to foreshadowing nearly all aspects of Mendelian genetics, he developed a theory 
of evolution based on mutation, selection, and geographic isolation. In this work 
he was so far ahead of his time that it is perhaps not surprising that his theories 
were not understood or appreciated. 

The evolutionary writings of Buff on (1707-1788), one of the most in- 
fluential biologists of the eighteenth century, have been variously interpreted — 
perhaps because they were so widely scattered among his extensive works. There 
can be little doubt that Buffon influenced the thinking of his successors about 
evolution, but it is not entirely clear whether he himself ever developed a con- 
sistent theory of evolution in which he believed wholeheartedly. One factor was 
his concern not to arouse the displeasure of the ecclesiastical authorities. How- 
ever, he did state parts of the theory of organic evolution in considerable detail, 
and his writings thus served as the starting point for much of the subsequent 
work. Among his contributions were several of significance. He anticipated 
Malthus, concerning the relation between population and food supply. He called 
attention to the fundamental similarities between animals of quite different 
species, thus giving impetus to the study of comparative anatomy, now a corner- 
stone in the evidence for evolution. His recognition of variation within species 
and of the possibility of gradual change within species giving rise to new 



20 • INTRODUCTION 

varieties seems very modern. The similarities between apes and men, the horse 
and the ass, made him raise the question of their relations to one another. His 
suggestion that the apes and the ass were degenerate types led to the idea of a 
common ancestry. He understood the significance of fossils and believed that the 
time scale needed to be greatly extended beyond the commonly accepted scale of 
his day. These and many other portions of his works indicate the modern lines 
along which his thinking was progressing. On the other hand, many passages 
could be cited to indicate that he believed in the immutability of species, a belief 
that grew from his use of hybrid sterility as the criterion for delimiting the 
species. Within the species, he thought change was possible, but, not visualizing 
a mechanism by which sterility might arise during evolution, he was more or less 
forced to argue against large-scale evolution. Buffon's writings contain contra- 
dictions, but they nevertheless were most influential in their impact on subse- 
quent generations. 

Going back in time, we find a number of speculative authors dealing 
with evolution, of whom we shall mention just one. De Maillet (1656-1738) in 
Telliamed drew together from the science of his day many threads to weave his 
theories. His unorthodox views were attributed to an Indian philosopher, "Telli- 
amed" (De Maillet spelled backward). Perhaps his major contribution was his 
clear statement on the nature and origin of fossils, about which varied opinions 
were still held. In his view, the gradual drying up of the seas over long periods 
of time was responsible for marine fossils in the mountains and could also ex- 
plain the similarities between aquatic and terrestrial forms, terrestrial species 
having been transformed from marine animals trapped in marshes. Many species 
undoubtedly failed to make the transition, he thought, but from the successful 
ones the land animals and birds arose. When he cited specific cases, however, he 
was not so cogent, for he derived birds from flying fish, and men and women 
from mermen and mermaids. Thus, he entangled facts with myths and legends, 
and his real contributions in the interpretation of fossils and rock stratification 
came under suspicion. 

The uniformitarianism of James Hutton (1726-1797) postulated that 
the ordinary forces of wind, water, heat, cold, and so forth, that we observe 
today are the same forces that worked to reshape and restructure the earth's sur- 
face in the past, and hence no mysterious or supernatural phenomena were in- 
volved in these changes. If this were the case, Hutton reasoned, the earth's age 
must be much greater than previously imagined and the various catastrophic 
theories must be wrong. William Smith (1769-1839) was primarily responsible 
for recognizing that each of the different layers or strata of rock has its own 
characteristic types of fossils and that the lower the strata, the less the fossils 
resemble living forms. Charles Lyell (1797-1875) in his Principles of Geology 
established the science of geology in its modern form. This work, published at 
the time of Darwin's voyage on the Beagle, was of great importance to the de- 



EVOLUTIONARY THOUGHT BEFORE DARWIN • 21 

velopment of Darwin's ideas. One of the major effects of the development of 
geology on the theory of evolution was that it showed the existence of a vast 
span of time during which evolution could have taken place. 

Erasmus Darwin (1731-1802), the grandfather of Charles, is note- 
worthy not only for that fact but also because in Zoonomia he gave the first clear 
statement of the theory of the inheritance of acquired characteristics, according 
to which the effects wrought by the environment on the organism are thought to 
be transmissible to the offspring. This theory was more completely developed 
by Lamarck (1744-1829), with whose name it is usually associated (Fig. 2-1). 




Lamarck's early years were spent in military service until ill health forced him to 
resign. An interest in botany, acquired while stationed in Monaco, led him to 
study medicine, of which botany was then an important part. A book on the flora 
of France established his reputation, won him the friendship of Buffon and other 
biologists, and eventually gained him a post as botanist at the Jardin du Roi. 
The reforms touched off by the French Revolution included the ouster of men 
who had previously been leaders in biology, and when two new chairs in zoology 
were created, the two most suitable candidates were Lamarck, a botanist nearing 
fifty, and St. Hilaire, a mineralogist. They apparently decided to split the animal 
kingdom between them, Lamarck taking the invertebrates and St. Hilaire the 



22 • INTRODUCTION 

vertebrates. The most remarkable aspect of this story is that both men went on to 
distinguished careers in their new fields. 

In Philosophie Zoologique (1809) Lamarck wrote more extensively 
about the evidence for evolution than had anyone prior to that time. His sug- 
gested mechanism for evolution was the inheritance of acquired characteristics. 
He believed that the activity of an animal enhanced the development of the more 
frequently used structures, producing modifications that were inherited; lack of 
use led to degenerative changes, which were also inherited. St. Hilaire, in sup- 
porting Lamarck, stressed the direct effects of the environment as causes of 
hereditary change, but Lamarck accepted this theory only in plants. An animal's 
need for a structure might also lead to its development — the long neck of a 
giraffe, for example, being the result of constant stretching over many genera- 
tions. Thus, use and disuse, need, and the direct effects of the environment have 
come to be considered as basic concepts in the theory of the inheritance of ac- 
quired characteristics. 

Unfortunately, despite its many appealing features, no critical evidence 
has ever been produced in favor of Lamarckianism. Nevertheless, this theory has 
been made the official theory of heredity in the Soviet Union under the name of 
Michurinism. The rise to power of Lysenko, which began in the early 1930s and 
became complete in 1948 with the abolition of teaching and research in Men- 
delian genetics, is a most unusual story. The attack was basically political, and 
the geneticists as well as their science were made to suffer. Despite its political 
success, Lamarck's theory of the inheritance of acquired characteristics still re- 
mains to be demonstrated experimentally, for Lysenko' s experiments lack ade- 
quate controls, do not involve strains of known ancestry, and are not treated 
statistically at all. 

Lamarck's ideas on evolution were subjected to forceful criticism by 
Cuvier (1769-1832), who was virtually a scientific dictator in France with un- 
paralleled political and scientific influence. Cuvier is generally considered to be 
the father of two sciences, paleontology and comparative anatomy. However, 
even though these two fields now furnish some of the most impressive evidence 
available on the course of evolution, Cuvier's work led him to believe in the 
fixity of species and to deny that evolution gave a satisfactory interpretation of 
his findings. He recognized that different rock strata contained different types of 
fossils, but attributed the gaps in the record to a series of catastrophes, following 
which immigration of different species from other areas repopulated the deva- 
stated regions. He believed the last such catastrophe to have been the flood 
recorded in Genesis. His followers carried his ideas one step further and postu- 
lated that successive creations were responsible for the new kinds of species 
found after each catastrophe. Although his active opposition to Lamarck and 
St. Hilaire certainly hampered the development and acceptance of the theory of 
evolution, nevertheless in one respect Cuvier was of great significance to subse- 



EVOLUTIONARY THOUGHT BEFORE DARWIN • 23 

quent work. St. Hilaire supported the concept of the unity of type among all 
animal species — the old idea of the scale of being or ladder of nature that can 
be traced all the way back to Aristotle. In particular, he compared the cephalopod 
mollusks, such as the squid, with the vertebrates. In the controversy that broke 
into the open between St. Hilaire and Cuvier in 1830, Cuvier conclusively dem- 
onstrated that no such unity existed and thus cleared the ground for the branch- 
ing system of divergent evolution. Whereas St. Hilaire (and Lamarck) were 
right in principle about evolution and wrong in detail, Cuvier was wrong in 
principle but right in detail about the data drawn from comparative anatomy. 
Since his views prevailed on both subjects, the evolution theory undeniably 
suffered. 

Thus the idea of evolution — that species change — was clearly not en- 
tirely original with Charles Darwin. Nor, as Darwin recorded in an introductory 
historical sketch to the Origin of Species, was he the first to propose the theory 
of natural selection as the mechanism of evolution. Several of his predecessors 
deserve mention. An expatriate royalist American physician, William Wells 
(1757-1817), appears to have been the first to enunciate the principle of natural 
selection in a reasonably modern form, in a paper entitled "An account of a 
white female, part of whose skin resembles that of a Negro" read in 1813 but 
generally ignored at the time. Another of Darwin's predecessors whom he also 
apparently overlooked was Patrick Matthew. In this case, Darwin could probably 
be excused, for Matthew's views on natural selection were published in the ap- 
pendix of a work entitled Naval Timber and Arboriculture. Yet Matthew, in his 
quest for recognition, called attention to his priority over Darwin in the title 
pages of his subsequent works. Recently, still another candidate for the honor of 
discovering natural selection has been unearthed in the person of Edward Blyth 
(1810-1873). It has been suggested that Darwin was less than completely candid 
in disclosing the extent of his debt to his predecessors, although to what extent 
this criticism is valid may be very difficult to determine. Even though it may be 
established that Darwin had read the papers of such men as Blyth and Matthew, 
it would be difficult if not impossible to learn whether he consciously drew on 
them at the time he achieved his great synthesis. Certainly his conduct toward 
Alfred Russell Wallace was always both proper and generous. 

The book The Vestiges of the Natural History of Creation was anony- 
mously published by Robert Chambers (1802-1871) in 1844 and went through 
ten editions in nine years. Since Chambers was an amateur scientist, his book 
was filled with errors, and scientists generally attacked it bitterly, an attack in 
which they were joined by the clergy. Their vehemence seemed to stimulate 
interest in the book rather than to kill it, however. The book showed that 
Chambers was familiar with the works of geologists such as Hutton and Smith 
and of such biologists as Buffon, Erasmus Darwin, Lamarck, St. Hilaire, and 
Cuvier. From them he drew his arguments in favor of cosmic and biological 



24 • INTRODUCTION 

evolution as opposed to special creation. The book was not significant for origi- 
nality but rather for the controversy and interest it aroused in the subject of evo- 
lution. Much of the ire that might have broken over Darwin's head had already 
been spent on Chambers. That the idea of evolution did not lack influential sup- 
port even in the 1850s just prior to publication of the Origin of Species is indi- 
cated by the 1852 essay of Herbert Spencer (1820-1903) called "The Develop- 
ment Hypothesis." In it for the first time the word "evolution" was used in the 
general sense in which it is used today. Thus it should be clear that the theories 
of Darwin and Wallace that struck with such impact in 1859 had a long period 
of development prior to the synthesis set forth in the Origin of Species. 

SUGGESTED READING 

Barlow, N., ed., 1958. The autobiography of Charles Darwin 1809-1882. London: 
Collins. 

Carter, G. S., 1957. A hundred years of evolution. London; Sidgwick and Jackson. 

Darwin, C, 1839. The voyage of the Beagle. New York: Bantam Books (1958). 

, 1872. On the origin of species. New York: Mentor Books (1958). 

, and A. R. Wallace, 1958. Evolution by natural selection. New York: Cam- 
bridge University Press. 

Eiseley, L., 1958. Darwin's century. Garden City, New York: Doubleday. 

, 1959. Charles Darwin, Edward Blyth, and the theory of natural selection. 

Proc. Amer. Philos. Soc. /03.'94-158. 

Glass, B., O. Temkin, and W. Straus, Jr., eds., 1959. Forerunners of Darwin. 1745- 
1859. Baltimore: Johns Hopkins University Press. 

Grant, V., 1956. "The development of a theory of heredity," Am. Scientist, 44:158- 
179. 

Greene, J. C, I960. The death of Adam. Ames: Iowa State College Press. 

Huxley, J. S., 1949. Soviet genetics and world science. London: Chatto and Windus. 

Irvine, W., 1955. Apes, angels, and Victorians. New York: McGraw-Hill. 

Lovejoy, H. O., 1953. The great chain of being. Cambridge, Massachusetts: Harvard 
University Press. 

Moore, R., 1953. Man, time, and fossils. New York: Knopf. 

Nordenskiold, E., 1928. The history of biology, L. B. Eyre, tr. New York: Knopf. 

Osborn, H. F., 1929. From the Greeks to Darwin, 2d ed. New York: Scribner's. 

Singer, C, 1959- A history of biology, 3d ed. New York: Abelard-Schuman. 



CHAPTER 



Darwin and 
after Darwin 



On February 12, 1809, two of the greatest figures of 
the nineteenth century were born, Abraham Lincoln and Charles 
Darwin. The circumstances surrounding the events could hardly 
have been less similar. Lincoln's start came in a backwoods log 
cabin, whereas Darwin was the son of a successful, well-to-do 
physician, Robert Darwin, who had married a girl of the famed 
Wedgewood pottery family. Thus, his family was doubly well off 
financially, and Charles later further insured his financial status 
by marrying his first cousin, another Wedgewood. As he put it in 
his autobiography, "I have had ample leisure from not having to 
earn my own bread." This, then, is one route to making great 
scientific discoveries, but it should be noted that the names of 
many others as well off financially as Darwin are now lost in 
obscurity. 

Darwin was quite a normal boy. He liked to fish and 
hunt, to collect almost anything, but not to attend school. His 
training at Dr. Butler's school consisted of classics exclusively, 
and he was considered by both his teachers and his father as a 
little below average in intelligence. His liking for mathematics 
and chemistry, and his interests in collecting insects and minerals 
were not satisfied in school. It seemed logical that he should fol- 
low in the footsteps of his father and grandfather before him and 
study medicine. For this purpose, he went to Edinburgh, but soon 
dropped this course of study. A major reason was his revulsion 



25 



26 • INTRODUCTION 

at some of the more gory and hideous scenes a medical man was expected to 
endure in those days before anesthesia. During his stay in Edinburgh he became 
acquainted with people who were interested in geology and natural history, and 
his own interests were aroused to the point where he took courses at the Univer- 
sity in these subjects. Unfortunately, as too often happens, formal instruction 
quickly killed this interest. 

His father, apparently fearing that his son was never going to amount 
to anything and seeking some sort of respectable career for him, then suggested 
that he go to Cambridge to study for the clergy. Charles was quite amenable to 
this suggestion, and went to Cambridge where, in due course, he received his 
degree, having achieved no particular distinction and having made no great 
efforts in his studies. In fact, most of his energies were devoted elsewhere, for 
he was an ardent hunter and horseman, and in the evenings, in a gentlemanly 
way, he sowed his wild oats, drinking and playing cards. Small wonder that his 
father thought that the cloak of respectability of a clergyman might help to keep 
his son from becoming a well-to-do ne'er-do-well. 

At this time his scientific inclinations were slightly manifest in his 
attendance at lectures in botany by Henslow, in his beetle collecting (he once 
was confronted by three unusual specimens and freed a hand to try for the third 
by tossing one into his mouth), and in his friendship with distinguished scien- 
tists such as Henslow and the geologist Sedgwick. This last aspect of his behavior 
was perhaps the most unusual. It is rather rare for a young college student to 
seek friendship among the professors, and it is perhaps even more rare to find 
the professors accepting as a friend one who had so far shown no particular 
promise. It is to their credit that the professors apparently saw something in him. 
Out of his friendship with the botanist Henslow came the event that changed 
and shaped the entire subsequent course of Darwin's life, for Henslow recom- 
mended him for the position of naturalist without pay on the Beagle (Fig. 3-1), 
a ship that was to make a long cruise around the world, charting many little- 
known areas (Fig. 3-2). After some discussion with his family, Charles accepted, 
and his career in the clergy was never again seriously considered. 

The voyage lasted five years, for the Beagle made many long stops, and 
much of the time was spent in South American waters. Darwin's account of his 
adventures, The Voyage of the Beagle, is a most fascinating and readable book, 
much more so than the closely argued Origin of Species. It is obvious that his 
experiences on this trip started the chain of thought that ultimately led to his 
theories of evolution. The course of his work gave him his first insight into the 
relations between species. He observed at first hand how species changed as one 
traveled from north to south in South America; he observed the character of 
island faunas; and he saw the relations between the fossils he discovered and the 
existing species in the same areas. He not only observed, but he made extensive 
and systematic collections of living and fossil materials. The facts of species 



DARWIN AND AFTER DARWIN • 27 



TV X 




Fig. 3-1. The Beagle, the vessel in which Charles Darwin sailed around the world. 



variation, of geographic distribution, and of the fossil record were almost forced 
to his attention. 

Unfortunately, the weak stomach that had contributed to the ending of 
his medical studies still plagued him, and he was seasick a good part of this five- 
year period. In fact, through the rest of his life, he was unable to stand any sort 
of excitement, for it almost inevitably led to digestive disturbances. Even having 
friends for dinner and a quiet talk afterward was enough to lead to discomfort 
and sleeplessness. A modern diagnosis would probably suggest that his troubles 
were psychosomatic, but nevertheless they were severe and sometimes incapaci- 
tated him for months at a time in later life. 

Upon his return to England in 1836, Darwin started to work on his 
collections and to write up the results of his travels. At the same time he began 
to collect all kinds of data bearing on the question of the transmutation of 
species. He carefully recorded all of the arguments both for and against, being 
especially careful to put down quickly those against, for he found that he could 
very conveniently forget them. In October 1838 he read for the first time 
Malthus' "Essay on Population," an excerpt from which is found in Ap- 



28 • INTRODUCTION 



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Cape • /*^D 

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Galapagos 










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Cape of 
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F*>. 3-2. The voyage of the Beagle (1831-1836) 



DARWIN AND AFTER DARWIN • 29 

pendix B. Here was a clue as to the mechanism by which species change. In 
Malthus' discussion of the reproductive potential of man being greater than the 
power of the earth to produce subsistence Darwin saw the essence of the struggle 
for existence that led to the theory of natural selection. 

He first wrote out his ideas on the origin of species in rough form in 
1842 and a more complete draft was drawn up in 1844, but he continued to 
assemble facts until 1856. Then, urged on by Lyell, he started to write up his 



MARINE IGUANA 




Chatham 



Charlt 



Hood 



Fig. 3-3. The Galapagos Islands and route of the Beagle. 



material in a work that he expected to fill four volumes. This undertaking was 
nowhere near completion when he received a manuscript from Alfred Russel 
Wallace, then in the Malay Archipelago, who asked him to read it and, if he 
thought well of it, to send it on to Lyell for his opinion. The paper contained, 
in complete detail, the theory of natural selection. 

The subsequent events tend to restore one's faith in human nature. 
Jealousy over priority among scientists is fairly common, yet the attitudes of 
Darwin and Wallace at this time and for the rest of their lives were exceedingly 
generous. Darwin sent the paper on with praise and the recommendation that it 
be published at once. Lyell and the botanist Hooker, aware of the long years 



30 • INTRODUCTION 

Darwin had spent in developing the theory, insisted that Wallace's paper and 
an extract of Darwin's manuscript and one of his letters written to the American 
botanist, Asa Gray of Harvard, should be published simultaneously. This was 
done, with the papers appearing in 1858. The projected four- volume work was 
abandoned by Darwin, who condensed his material into a single volume, the 
famed Origin of Species, which appeared the following year. This work was an 
immediate success and had terrific impact not only on the scientific world but on 
the world at large, in contrast to the reception of the original papers. 




Fig. 3-4. Charles Darwin in 1840, two years 

prior to his first draft of the theory of evolution 

by natural selection. (From a water color by 

George Richmond.) 

The circumstances under which Wallace arrived at the theory of natural 
selection were rather similar to those that initiated Darwin's trend of thought. 
Wallace was a naturalist whose travels among the islands of the East Indies 
impressed on him the differences between species as well as their obvious rela- 
tionships to each other, and led him to evolution and natural selection as the 
explanation for his observations. It seems as if biological knowledge had reached 
the point where an adequate training and extensive field work led almost in- 
evitably to the major synthesis that Darwin and Wallace achieved independently. 

In his book Darwin actually presented evidence bearing on two distinct 
subjects: the theory of evolution, and a theory of the mechanism of evolution — 



DARWIN AND AFTER DARWIN • 31 

that is, natural selection. Darwin proposed on the one hand that evolution had 
occurred, that existing species are descended from similar but somewhat different 
species that lived in the past. The evidence he presented came from his study of 
variation under domestication and in nature, from taxonomy, from comparative 
anatomy and embryology, from the geographical distribution of species, and 
from the geological record. His presentation is still one of the finest arguments 
for evolution. He also proposed natural selection as the mechanism making evo- 
lution possible. It should be noted that evolution could still be valid even if, as 
now seems very unlikely, the theory of natural selection were shown to be false. 

The theory of natural selection is based upon a few, simple, easily veri- 
fied observations and the conclusions to be drawn from them. It can readily be 
observed that the reproductive potential of all species is far greater than is re- 
quired to replace the existing population, the possible rate of increase forming a 
geometrical progression. Even elephants, presumably the slowest breeders of all, 
were shown by Darwin to have this great potential. He estimated that from one 
pair, breeding from age 30 to 90 and having only six young in this span, there 
would be descended a living population of 19,000,000 after 750 years. The 
spread of the English sparrow and the starling after their introduction into the 
United States in small numbers less than a century ago is further evidence of the 
tremendous reproductive capacity of all species, which is only realized under the 
most favorable conditions. 

Despite this reproductive potential, however, it can easily be verified 
that the population size of any species in a given area is relatively constant. 
Fluctuations occur from year to year, but ordinarily there is no continuous 
increase. 

The obvious conclusion from these two observations is that not all of 
the progeny produced by any generation reach maturity, but that many die during 
the early stages of the life cycle. 

The third observation by Darwin was that variation is a universal phe- 
nomenon, that no two individuals are ever exactly alike. 

Darwin's final conclusion, then, was that, since individuals differ from 
each other, some will inevitably be better adapted to survive under the existing 
conditions than others. Since a large proportion of each generation dies before 
reaching maturity, the better adapted individuals will tend to survive while the 
less well adapted will die. Even though most of the deaths occur at random, if 
this differential affects the survival of the remainder, it will still be significant 
although more difficult to detect. Finally, if the adaptive traits are hereditary, the 
survivors, who become the progenitors of the next generation, will tend to trans- 
mit their favorable traits to their offspring. Therefore, the next generation will 
have a higher proportion of well-adapted individuals than the previous one. 
Hence, in time, this natural selective process will change the average character- 
istics of a species, and evolution will occur. 



32 • INTRODUCTION 

The Origin of Species was widely read and discussed as soon as it ap- 
peared. Controversies arose over the validity of the theories of evolution and 
natural selection. Powerful forces in the church, the most eminent being Bishop 
Wilberforce, attacked the book, but there were also eminent scientists such as the 
anatomist Richard Owen and the Swiss American zoologist Louis Agassiz who 
did not accept its conclusions. The distinguished German embryologist von Baer 
accepted evolution but rejected natural selection, for he did not accept the idea 
of a completely materialistic system. The strongest advocate of Darwin's views, 
since his health limited his participation in the public discussions stimulated by 
his book, was Thomas Henry Huxley. In lectures, articles, and debates Huxley 
educated the world on the significance of these theories. The acceptance of 
Darwin's views came quite rapidly among the scientists, but somewhat more 
slowly by the general public. Owen rather weakened his case in opposition when 
it was discovered that anonymous articles, attacking Darwinism and citing the 
eminent authority, Dr. Richard Owen, had actually been written by Owen him- 
self. The position of the church was modified, in part at least, as the result of 
the famous debate between Bishop Wilberforce and Huxley in which Huxley 
won a decisive victory. When Bishop Wilberforce "begged to know, was it 
through his grandfather or his grandmother that he claimed his descent from a 
monkey?" Huxley replied that he would not be ashamed to have a monkey for 
an ancestor, but he would be "ashamed to be connected with a man who used 
great gifts to obscure the truth," and with this stirring statement, he won the 
day. A rather similar debate took place at Harvard between Asa Gray and Louis 
Agassiz. With the support of such distinguished advocates as Lyell, Huxley, and 
Hooker in England, Asa Gray in America, and Haeckel, an embryologist, and 
Gegenbauer, a comparative anatomist in Germany, Darwin's theories within a 
very few years gained a strong foothold in the world of ideas. 

The effects of Darwin's theories on biology were far reaching. System- 
atics received a great stimulus, for now the rationale behind classification was 
the actual relationship among the different species. Systematics became the study 
of evolution. Similarly, comparative embryology and comparative anatomy under- 
went rapid growth as their value in the working out of phylogenies became 
apparent. Paleontology, of course, as the main source of information about the 
past history of living things, also received a great impetus. Other fields were 
influenced to greater or lesser degrees, but none remained untouched. 

In the years just after the book was published, the major advances were 
made in disentangling the phylogenetic threads. The theory of natural selection 
was accepted by the adherents of Darwinism and condemned by its opponents, 
both without much evidence. The major weakness in the theory of natural selec- 
tion was the lack of understanding of variation and its mode of transmission 
from one generation to the next. Darwin recognized this weakness better perhaps 
than most of his adherents. The basic principles of heredity were known, but 



DARWIN AND AFTER DARWIN • 33 

they were understood apparently only by their discoverer, Mendel; others who 
knew of his work either failed to understand it or else failed to appreciate its 
significance. Darwin, who might have been the one person capable of appre- 
ciating Mendel's work, never became cognizant of it. It is interesting to speculate 
what the course of events might have been if Mendel had written to Darwin of 
his results. But it never happened, and Mendel's work lay neglected from 1865 
to 1900. 

However, progress was being made in still another area of biology, the 
study of the cell, particularly by Strasburger and Flemming. The details of the 
structure and behavior of the various parts of the cell were worked out in the 
closing years of the nineteenth century. In particular, the chromosomes were 
identified, and the details of their behavior during cell division and gameto- 
genesis were scrutinized. Cytology became a separate branch of biology. A syn- 
thesis of much of this work was undertaken by Weismann, who realized that 
the hereditary material must reside in the nucleus on the chromosomes, and who 
also originated the "germ line" theory. This theory pointed out that the germ 
cells are set aside very early in development and are uninfluenced by the rest of 
the cells in the body, the somatic cells. Under this theory, the inheritance of 
acquired characteristics would be impossible. Furthermore, the suggested mech- 
anism for such inheritance, Darwin's theory of pangenesis, was outmoded. The 
pangenes had been visualized as being formed in all parts of the body and, 
bearing the traits exhibited there, coming together to form the gametes. There is 
no evidence for this theory proposed by Darwin. 

In 1900, Correns, de Vries, and von Tschermak independently discov- 
ered Mendel's paper, after essentially reaching Mendel's results, and the new 
science of genetics finally was born. Mendel's laws were a major step forward in 
the understanding of variation. They showed that variations were inherited in a. 
particulate fashion, and that .blending inheritance, visualized by Darwin, did not 
occur. Hence, variability is not lost in crossing, but rather, as Hardy and Wein- 
berg independently suggested in 1908, tends to remain constant in a population. 
Furthermore, Mendel's work led to an understanding of the way in which the 
recombination of characters could occur with the consequent new variations. 

The rise of genetics, despite its contributions to the understanding of 
variation, was followed by a general eclipse of the theory of natural selection as 
the mechanism of evolution. Its place was taken by the mutation theory of de 
Vries, proposed in 1902. In his work with the evening primrose, Oenothera, 
de Vries occasionally found sports — that is, distinctly different types of plants, 
now known by the more pedestrian term "mutations." He therefore proposed 
that evolution was not due to the gradual accumulation of numerous small 
changes by natural selection, but instead occurred as the result of large jumps 
made possible by mutations of the type he was discovering. This theory won 
wide support among early geneticists, for the variations familiar to them in 



34 • INTRODUCTION 

their work were of this type, and did not conform at all to Darwin's concept. 
Thus, such eminent geneticists as William Bateson and Thomas Hunt Morgan 
led the way in the early years of the century in rejecting natural selection, and 
many others concurred. Ironically, most of de Vries' mutations were later demon- 
strated to be due to chromosomal changes rather than to changes in the genes 
themselves, and hence were not mutations in the usual restricted sense at all. 

Still further reason to doubt Darwin's theory came with Johannsen's 
demonstration, in 1910, that selection was effective only in genetically hetero- 
geneous populations and was completely without effect on environmental varia- 
tions. Darwin's failure to distinguish clearly between hereditary and environ- 
mental variation and his acceptance of Lamarckianism were thus shown decisively 
to be in error. 

Not all biologists followed the lead of the geneticists. Many felt, as the 
paleontologist Simpson puts it, "that a geneticist was a person who shut himself 
in a room, pulled down the shades, watched small flies disporting themselves in 
milk bottles, and thought that he was studying nature." The studies of the fossil 
record revealed, where the evidence was complete, that evolutionary changes 
had been gradual rather than abrupt. Taxonomists, in their work with living 
species, found that the different species and subspecies differed from each other 
in numerous minor quantitative traits rather than in a few major characteristics. 
Furthermore, a group of students of heredity who worked with continuously 
varying traits rather than the alternative traits so commonly studied by Mendel ian 
methods obtained results more in keeping with Darwin's ideas than those of the 
new Mendelian genetics. This group had its origin with Galton, Darwin's first 
cousin, well before 1900, and was responsible for the development of the science 
of biometry. Karl Pearson was the biometrician who came most directly into con- 
flict with the early Mendelians, led in England by Bateson. Neither side recog- 
nized any merit in the work of the other group. Feelings ran so high that 
Bateson, in order to get his experimental results into print, had to start his own 
journal. However, people such as the paleontologists, taxonomists, and bio- 
metricians who continued to believe in natural selection were frequently regarded 
as out-of-date die-hards. 

A major advance was made when it was shown that continuous varia- 
tion had a Mendelian basis. Thus a reconciliation was possible between the 
Mendelians and the followers of Pearson. Since then, there has come about a 
synthesis leading to an evolutionary theory that is now generally accepted among 
paleontologists, systematists, geneticists, and most other biologists. Underlying 
this new synthesis is the increased knowledge and understanding of variation. 
Morgan and his co-workers conclusively demonstrated that the Mendelian factors 
or genes were located on the chromosomes, and thus established not only the 
physical basis of heredity but of evolution. Our understanding of the nature of 
mutation and of the mutation process has greatly increased, notable advances 



DARWIN AND AFTER DARWIN • 35 

being Muller's induction of mutations with x-rays in 1927, and the more recent 
success of chemical mutagens, first demonstrated by Auerbach. The direct appli- 
cation of genetic knowledge to evolutionary problems was made possible by the 
theoretical development by Fisher, Haldane, Tchetverikov, and Wright of popu- 
lation genetics. As a result of their efforts, evolutionary change has come to be 
recognized as the result of the combined effects of several forces on the fre- 
quencies of the genes in breeding populations. One of these forces is natural 
selection, which remains as a cornerstone to an expanded and strengthened 
theory of the mechanism of evolution. The modern synthesis or Neo-Darwinism, 
as it is often called, has been largely responsible for the renewed interest in 
evolutionary problems. 

SUGGESTED READING 

See references at the end of Chapter 2. 




PART 



n 



The Evidence 
for Evolution 



CHAPTER 



4 

The Fossil Record 



Living species, by their very existence, pose the questions 
that the theory of evolution attempts to answer, but the fossil 
record is another material source from which information and in- 
sight can be derived. Few people have ever tried to deny the 
existence of living species, but many interpretations have been 
made of the fossils that have been found all over the world. 
These interpretations now have passed into the realm of myths, 
and fossils are generally accepted for what they are, the remains or 
traces of previously existing animals and plants preserved in the 
earth's crust. The fossil record, unfortunately, is incomplete, but 
the reasons for the gaps in the record will become clear from a 
knowledge of the nature of fossils and the conditions necessary 
for their formation. The two conditions under which a fossil is 
generally formed from a living organism are that it have some 
hard parts, and that it be buried quickly in some protecting 
medium. Quick burial tends to retard or prevent the decomposi- 
tion of the organisms by solution or oxidation or bacterial action. 
Fossils have been formed in such places as the floors of caves, in 
tarpits and oil seeps, in bogs and quicksand, and under volcanic 
ash or windblown sands, but the great majority have been covered 
over by water-borne sediments. 

A fossil may be anything from an intact woolly mam- 
moth frozen in the Siberian tundra to the footprint of a dinosaur. 
Complete organisms, however, are very rare, and even unchanged 
hard parts, such as bone, shell, or woody tissue, are uncommon. 
Usually the fossil has undergone some change, with the original 



39 



40 • THE EVIDENCE FOR EVOLUTION 

hard parts having gradually been replaced by some mineral substance such as 
calcium carbonate, silica, or iron pyrite. This particle-by-particle replacement is 
so slow that the microscopic structure of the hard parts is preserved, and the cell 
walls of wood, for example, can still be studied even though the organic matter 
is completely gone. In some cases, however, especially in plants, the more volatile 
elements may be distilled off, leaving behind them a carbon residue. If the 
original hard parts are dissolved, a "mold" of the shape may then be left in the 
surrounding rock. If the mold is subsequently filled by a foreign mineral sub- 
stance, such as quartz, a "cast" is formed. The cast, of course, retains no indica- 
tions of the original microscopic structure. 

The normal habitat of many species has undoubtedly precluded their 
appearance in the fossil record simply because conditions were unsuitable for 
fossil formation, as in the deep seas or high uplands, for example. Even if 
buried, the organism needs hard parts, for otherwise the chances of preservation 
are very slight. Whole groups of species may be virtually absent from the fossil 
record because they did not meet these requirements. The fossil record is there- 
fore by no means a random sample of all previously existing species, but a 
specially selected group. From the nature of the record, it is obvious that it will 
never be complete, although subsequent finds will tend always to narrow the 
gaps and to supply the "missing links." 

In addition to the information about life in the past, fossils reveal still 
other facts about past conditions. The discovery of the fossil remains of marine 
organisms like corals and sea urchins far inland in Indiana or 20,000 feet up in 
the Himalayas has far-reaching geological implications, for at one time Indiana 
must have been covered by the ocean, as were the Himalayas, which were subse- 
quently thrust up to their present towering heights. Fossil palms and alligators 
in the Dakotas and musk oxen in Arkansas are indicative of wide fluctuations in 
past climatic conditions. 

Reconstructing the Past 

Perhaps the greatest accomplishment of the paleontologists has been 
their reconstruction of the sequence of past events. Water-borne sediments are 
deposited in layers or strata that are then, through pressure, converted to rock. 
Undisturbed deposition over a long period of time has thus given rise to an 
accumulation of sediments many feet thick, with the oldest deposits at the bottom 
and the most recent at the top. The fossils in the bottom layers must, therefore, 
represent the oldest species. If it were possible to find a place where deposition 
of sediments had been continuous since the formation of the earth in its present 
structure, the strata would form a complete geological column, and the included 
fossils would furnish a fairly good record of the forms of life that had existed 
during this period. Although some deposits are thousands of feet thick, no such 



THE FOSSIL RECORD • 4l 

complete geological column is known. Such thicknesses, built up very gradually, 
give some appreciation of the vastness of geological time, yet they represent only 
small fractions of the total. In a given bed of sedimentary rock, the fossils in 
different strata are different from one another, but the fossils in adjacent layers 
are more alike than those further removed. The more recently formed fossils 
show greater similarity to existing species than those in the lower strata. The 
presence of the same types of fossils in deposits in different parts of the world 
has been assumed, as seems reasonable, to indicate that these sediments were 
laid down at approximately the same time. On this basis, it has been possible to 
correlate the deposits all over the world into one chronological series, and a 
geological column has been constructed through these correlations. Thus has the 
earth's history been reconstructed. New finds can be fitted into the rest of the 
record, but the dating is relative rather than absolute. The absolute age, which is 
obtained from studies of radioactive minerals, has been estimated as about 4.5 
billion years. Though rocks apparently bearing fossils of algae, protozoans, and 
fungus spores have been estimated to be as old as 3.3 billion years, the record 
was very fragmentary up until about 500 million years ago. Some of the major 
subdivisions of geological time are shown in Table 4-1. Though the major phyla 
have been represented in the fossil record ever since the Paleozoic, the species 
representing each phylum have changed considerably with the passage of time. 



TABLE 4- 1 

The Geological Time Scale (After Kulp) 









Time estimated in 


Era 


Period 


Epoch 


millions oi 

Since 
Beginning 


years 
Duration 


Cenozoic 


Quaternary 


Recent 


.011 


.011 


(Age of Mammals) 




Pleistocene 


1 


1 




Tertiary 


Pliocene 


13 


12 






Miocene 


25 


12 






Oligocene 


36 


11 






Eocene 


58 


22 






Paleocene 


63 


5 


Mesozoic 


Cretaceous 




135 


72 


(Age of Reptiles) 


Jurassic 




181 


46 




Triassic 




230 


49 


Paleozoic 


Permian 




280 


50 


(Age of Fishes) 


Pennsylvanian 




310 


30 




Mississippian 


i 


345 


35 




Devonian 




405 


60 




Silurian 




42 5 


20 




Ordovician 




500 


75 




Cambrian 




600 


100 


Precambrian 






5,000± 


4,400± 



42 • THE EVIDENCE FOR EVOLUTION 

The Paleozoic, for example, is known as the Age of Fishes, the Mesozoic as the 
Age of Reptiles, and the Cenozoic as the Age of Mammals; the mammals first 
appeared in the fossil record during the late Mesozoic but reached their climax 
only during the Cenozoic. 

Even as recently as the Mesozoic era, practically no living species ex- 
isted. Many species have appeared in the fossil record, persisted in it for varying 
periods, and then disappeared. Where the record is fairly complete, gradual 
changes within a given group can be followed from the older to the more 
recent strata. The evidence shows that distinct new species have appeared in all 
parts of the world throughout geological time. There is no time or place, ap- 
parently, at which new species could not have originated. The most reasonable 
and complete explanation for the evidence from the rocks — physical evidence 
that can hardly be ignored — is the theory of evolution; that is, living species, 
through a series of gradual changes, have descended from somewhat different 
species living in the past. Today, in fact, we think of the fossil record in terms 
of evolution to such a degree that it is hard to separate the record from its inter- 
pretation. Yet Cuvier, in Lamarck's time, and Louis Agassiz in Darwin's, prob- 
ably the leading paleontologists of their day, both opposed the theory of evolu- 
tion, using the paleontological materials to support their arguments. Since then, 
however, our vastly increased knowledge about paleontology, due in large part 
to the stimulus of Darwin's theories, has made it one of the bulwarks of proof 
that evolution has actually occurred. 

Extinction and Evolution 

Practically all of the species recognized from fossils no longer exist. 
There are two routes to extinction — one leading to complete extinction; the 
other, through evolutionary change, to new species. The evolution of new species 
may take place in two ways. One is a transformation in time, species A evolving 
into B, B into C, and so on as time passes. The other is a multiplication of 
species in space, two species, B and C, originating simultaneously from a single 
species, A. Because of this latter process (now usually referred to as speciation, 
in a restricted sense of the word), the number of coexisting species has tended 
to increase as more and more of the available ecological niches have been occu- 
pied. For example, invasion of the land did not occur until plant and animal 
species adapted to life on land had evolved from the ancient aquatic types. A 
whole new range of possibilities then opened up, and adaptive radiation of 
species from these first successful invaders of the land into a variety of diverse 
habitats occurred. Because the process of adaptive radiation through speciation 
has continued through geological time, the number of living species is probably 
greater today than it has been at any time in the past. 

Evolutionary changes are gradual, with no positive evidence for the 



THE FOSSIL RECORD • 43 

formation of species by a cataclysmic process or saltation existing in the fossil 
record, but all evolutionary rates are not the same. Different groups may have 
different average rates of evolution; the mammals, for example, appear to have 
evolved much more rapidly than the ammonites. Even within a single group, the 
rate of evolution may change from one time to another. Though evolution goes 
on between generations rather than within generations, nevertheless generation 
length seems to have no relation to evolutionary rates, for the mammals, with a 
very long generation length, have had an extremely rapid rate of evolution. 

Frequently, the slow, steady evolution within a particular evolutionary 
line shows a series of changes in a single direction or a trend, a type of evolu- 
tion known as "orthogenesis." Because such trends are so common, it has been 
suggested that evolution may have a sort of momentum, which causes it, once 
under way, to continue to move in the same direction, even when the changes 
are no longer adaptive. It has been suggested, for example, that the Irish elk 
became extinct when its massive antlers became so heavy that the animals could 
no longer hold up their heads or else snagged them in the brush and thus starved 
to death. The saber-toothed tiger was supposed to have met a similar fate when 
his fearsome fangs became so long that he could no longer get any food past 
them. However, more thorough study has shown that the trends are due to con- 
stant selection pressure in a given direction, and that the changes are adaptive; 
hence the term "orthoselection" would be more descriptive than orthogenesis. 
Whatever the causes of extinction for the Irish elk and the saber-toothed tiger, 
they were not carried off by runaway evolution. 

One implication of orthogenesis, divorced as it is from adaptation, is 
that there is a vital force or elan vital animating all living things. In addition, 
the prevalence of evolutionary trends has led to speculation that evolution is 
directed toward some ultimate goal, a concept known as "finalism." There is no 
reason or need, however, to invoke either vitalism or finalism to account for the 
evidence. 

Major adaptive shifts, giving rise to new and distinctive groups, repre- 
sent changes in the direction of evolution and usually a change in rate as well. 
The gaps in the fossil record usually seem to occur at the crucial stages where, 
if evolution is a gradual process, transitional forms connecting major groups 
ought to be found. Failure to find many transitional fossils has led many author- 
ities to postulate a different evolutionary mechanism for the origin of higher 
taxonomic groups, but our subsequent discussion will show that no special 
mechanism is demanded by the evidence. 

In discussions of trends in evolution, the terms "generalized" and "spe- 
cialized" are frequently used, often with the corollary that "specialization is the 
prelude to extinction." Such a generalization is unwarranted. The terms "gen- 
eralized" and "specialized" have meaning only in a relative and rather limited 
sense, though they can be useful. To raise a specific question, were the early 



44 • THE EVIDENCE FOR EVOLUTION 

mammals specialized or generalized? Had a zoologist of the day (if such existed) 
compared them with their contemporaries, the dominant reptilian group, they 
might well have been considered a small, specialized, and rather aberrant group 
of reptiles, destined therefore to rapid extinction. In this instance specialization 
was a prelude to new evolutionary opportunities. Compared with recent mam- 
mals, however, these early mammals must be considered quite generalized. A 
rather similar verbal pitfall is found in the use of the terms "primitive" and 
"modern" species. The shark and the frog, for instance, are often cited as ex- 
amples of primitive vertebrates, with the mammals held up as the modern type. 
Since sharks, frogs, and mammals are all living today, one group is just as old as 
the other, and the ancestry of one can be traced back just as far as that of 
another, though a greater variety of ancestors may appear in one lineage. The 
fallacy would be even clearer if, through some quirk of fate, all mammals be- 
came extinct. If used with reference to time of origin, however, the terms can 
be useful and not especially confusing. 

Vertebrate Evolution 

In order to give some appreciation of the type of information available 
in the fossil record, the history of the vertebrates or backboned animals (the 
subphylum Vertebrata of the phylum Chordata) will be outlined (see Fig. 4-1). 
The first vertebrate fossils appeared in the Ordovician period of the Paleozoic 
era, which began about 425 million years ago. These fishlike animals were small, 
armored, bottom dwellers, but lacked both jaws and paired fins. Known as 
ostracoderms, they belonged to the class Agnatha, which today is represented by 
just a few surviving species, the most familiar being the lampreys. The Agnatha 
remained common throughout the Silurian and Devonian periods. The first 
vertebrates to have jaws and paired appendages appeared among the late Silurian 
fossils, were very common in the Devonian (325 million years ago), and had 
virtually disappeared from the Mississippian record. This class of early fishes, the 
Placodermi, is now extinct. The Chondrichthyes, a group to which the present- 
day sharks and rays belong, first appeared in the middle and late Devonian, 
became abundant in the Mississippian and Pennsylvanian, and have remained 
common up to the present day. At about the same time the bony fishes 
(Osteichthyes) appeared in the fossil record and have flourished ever since. 
Unlike the sharks, they had a specialized spiracle, an added (hyoid) support for 
the jaws, and an air bladder or lungs. They include two major groups, the 
Choanichthyes, including the lobe-finned fishes or crossopterygians and the living 
lung fishes or Dipnoi, and the Actinopterygii, or ray-finned fishes, to which 
belong more than 90 percent of the existing species of fish. 

The first land vertebrates, with legs and lungs, did not appear as fossils 
until the late Devonian. These first tetrapods were amphibians, a group that had 



THE FOSSIL RECORD • 45 



Periods 
Cretaceous 

Jurassic 

Triassic 
Permian 

Pennsylvanian 

» 

Mississippian 

Devonian 

Silurian 




PLACODERMI / 



Ordovician 



Fig. 4-1. The phylogeny of the vertebrates. (After Romer). 

its heyday during the Mississippian and Pennsylvanian periods and has since 
been a subordinate part of the land vertebrate fauna, represented today by the 
frogs, toads, and salamanders. The first known reptiles were found in rocks of 
Pennsylvanian origin. Though it is a relatively simple matter to distinguish be- 
tween recent amphibians and reptiles, the criteria tend to break down for the 
ancient species. One reason for this difficulty is that the most significant differ- 
ence between the amphibians and reptiles lies in their modes of reproduction and 
development. The reptiles were completely freed from dependence on an 



46 • THE EVIDENCE FOR EVOLUTION 

aquatic environment at any stage of their life cycle because their shelled eggs 
could develop on land. The amphibian egg, with little yolk, must be laid in the 
water, and the young tadpoles, a larval stage, soon emerge. The developing 
embryo of the reptilian egg is bathed in fluid, too, but the fluid is contained in 
a sac, the amnion, which encloses the embryo. Another membranous sac, the 
allantois, serves as a respiratory structure for gaseous exchange and also as a 
storage place for excretory wastes. Because of the large yolk supply, the young 
reptiles develop much further than the amphibians before they hatch from the 
egg. The reptiles increased in numbers during the Permian and were the domi- 
nant land vertebrates throughout the Mesozoic era. Many of the reptilian groups 
then prominent, such as the dinosaurs, ichthyosaurs, mosasaurs, and plesiosaurs, 
are now extinct, and the reptiles today are represented by such groups as the 
snakes, turtles, alligators, and lizards. 

The first birds (Aves) appeared in the fossil record in the Jurassic 
period of the Mesozoic, but unlike modern birds, which did not appear until 
the Cenozoic, they had teeth and a tail composed of vertebrae, and were difficult 
to distinguish from reptiles. Even today birds seem much like glorified reptiles. 

Though mammallike reptiles (Therapsids) existed in the late Paleozoic, 
the first true mammals did not appear as fossils until the Triassic and they did 
not form an important part of the fauna until the Cenozoic. The mammals are 
characterized by the presence of mammary glands, hair, warm blood, and a rela- 
tively large brain, which is probably in large measure responsible for their cur- 
rent dominance. Though most mammals bear living young, which have under- 
gone development in the uterus of the mother while nourished via the placenta, 
some living mammals, such as the duck-billed platypus, lay shelled eggs. This 
group, the monotremes or Protheria, is apparently quite distantly related to the 
mammalian lines of descent that gave rise to the marsupials (Metatheria) and 
the placental mammals (Eutheria). The first fossils that show clearly human 
affinities appeared in the fossil record less than two million years ago. 

Evolution of the Horse 

Horses have left behind the most complete sequence of fossils yet dis- 
covered. Their history has therefore been worked out in greater detail than that 
of any other group (see Fig. 4-2). Man and the horse have been closely asso- 
ciated for centuries, but whereas the human fossil record has been traced back 
to something less than two million years, fossil horses first make an appearance 
in the early Eocene some sixty million years ago. Although no direct links with 
animals living in the Paleocene are known, the indications are that the horses, or 
Equidae, are descended from the order Condylarthra, an order of five-toed 
hoofed mammals or ungulates that is now extinct. Horses belong to the order of 
odd-toed ungulates, the Perissodactyla, and number among their relatives the 



THE FOSSIL RECORD • 47 



SOUTH AMERICA 



NORTH AMERICA 



OLO WORLD 




Fig. 4-2. The evolution of the horse. (With permission of Simpson.) 



living rhinoceroses and tapirs and the extinct chalicotheres with clawed feet and 
the enormous "horned" brontotheres. 

The primary center for horse evolution was in North America, especially 
in the Great Plains region, for the most abundant and continuous fossil record 
has been found there. From time to time some of the species spread to the Old 
World, but not until a land connection was again established at the end of the 
Pliocene were they able to reach South America. Consequently the fossil record 
of the horses on that continent is confined to Pleistocene deposits. 



48 • THE EVIDENCE FOR EVOLUTION 

The earliest Eocene equines were so unlike the modern horses that they 
were called Hyracotherium because of their rodentlike appearance. They later 
became known as eohippus, the dawn horse. Eohippus, from which all subse- 
quent horse evolution proceeded, was a small, browsing animal the size of a fox 
terrier and standing only ten to twenty inches tall at the shoulder. His back was 
arched, and his hind legs and tail were relatively long. His front feet each had 
four toes, the hind feet only three, and even though tiny hoofs were present, 
most of the weight was born by pads. 

Miohippus from the Oligocene was the first horse with three toes on all 
feet, but the lateral toes were still functional. About the size of a sheep, this 
browsing horse was apparently more intelligent and fleet of foot than its 
predecessors. 

The fossils of Merychippus come primarily from the Miocene. This 
group of horses had high-crowned teeth, adapted for grazing on the relatively 
harsh grasses of an open prairie habitat, rather than the low-crowned teeth of its 
predecessors, which were adapted to browsing on succulent shoots and leaves. 
Furthermore, though Merychippus still had three toes, the outer toes were re- 
duced, barely touching the ground, and the leg had become, with its well- 
developed cannon bone, an efficient spring mechanism. This group, which 
marked the completion of the transition from browsing to grazing, was highly 
successful, numerous, and widespread. Pliohippus, the first one-toed horse, ini- 
tially appeared in the Pliocene deposits. The two slender splint bones on each side 
of the cannon bone are the only vestiges of the other two toes. Pliohippus was 
succeeded in the Pleistocene by members of the genus Equus to which belong 
all the living Equidae — the horses, zebras, asses, and onagers. 

In outline, the material just presented indicates the line of succession 
from the earliest known equids up to the present-day horses. The abundant 
fossils have made it possible to document the changes rather than having to 
attempt to fill gaps in the record by speculation or conjecture. The major 
changes from Hyracotherium to Equus were an increase in overall size, a reduc- 
tion in the number of toes, a transition from browsing to grazing, and the asso- 
ciated increase in the height and complexity of the teeth. 

To present this record without additional information, however, is to 
give a greatly oversimplified conception of how evolution actually took place in 
the horse. As presented, it appears to have been a linear process, perhaps with 
overtones of orthogenesis. Actually, this was far from the case. At each level 
from eohippus on, an adaptive radiation took place and numerous groups 
evolved, all of which except Equus are now extinct. Miohippus, for example, 
was ancestral not only to Merychippus, which completed the transition to graz- 
ing, but also to a line that culminated in the large three-toed browsing "forest" 
horses known as Hypohippus, and to at least three other distinct lineages. 
Similarly, Merychippus, successfully adapted to grazing, became the source of a 



THE FOSSIL RECORD • 49 

number of three-toed grazing horses such as the highly successful genus Hip- 
parion, as well as of the one-toed group Pliohippus. Pliohippus gave rise not 
only to Equus but also to the genus Hippidion, which reached South America in 
the early Pleistocene and there underwent adaptive radiation. Therefore, before 
reading any trends into the record, we must try to see whether they really exist. 
For example, horses in some cases did increase in size, but some lines remained 
essentially unchanged for long periods, and in others actual decreases in size 
occurred. The reduction in numbers of toes was by no means universal nor was 
it a gradual, inexorable process. The change from four front toes to three oc- 
curred in a relatively short period and was followed much later by the rapid 
transition from three toes to one. In each case it was an adaptive shift occurring 
in one among a number of existing groups. Finally, the change from low- 
crowned to high-crowned, more complex teeth was one adaptive shift in the 
evolution of the browsing horses that happened to be highly successful because 
it opened up a new ecological niche to exploitation. However, other types of 
trends can also be traced in the evolution of the teeth of browsing horses. Thus, 
this brief resume indicates that both the rate and the direction of evolution may 
change and that the changes seem to be related to adaptation. Only so long as 
an evolutionary shift continues to bring improved adaptation will it continue. 
To this extent, evolutionary trends may be observed, but they are due to natural 
selection, not to orthogenesis impelled by some mysterious internal force. The 
most persistent trends would be expected in the improvement of those traits 
that confer adaptive advantage in any kind of environment. 

Several obvious facts stand out from this brief review of the verte- 
brates' history. Not all of the major groups of vertebrates have been represented 
since the Ordovician; instead, new groups have appeared periodically. The more 
recent deposits contain vertebrates much more like living species than the most 
ancient fossils. Great numbers of species found as fossils have become extinct. 
Though gaps exist in the record, types intermediate between the major groups 
have been discovered. The most far-reaching and consistent explanation of the 
vast array of facts accumulated from the study of paleontology is the theory of 
evolution. The sequence of appearance in the rock strata depicts the phylogeny 
of the group (see Fig. 4-1). A major advance in the course of vertebrate evolu- 
tion and hence of human evolution — for man fits into the overall scheme — was 
the acquisition of jaws and paired appendages by the Placodermi; another such 
advance occurred when the lobe-finned fishes gave rise to the four-footed 
amphibians, which breathed air with lungs derived from the air sacs or lungs of 
the fishes. Man and the dog and the horse show so many similarities — that com- 
plex of traits characteristic of placental mammals — because they had a common 
ancestry up until about 75 to 100 million years ago. 

Although speculation as to what follows in vertebrate evolution is the 
next logical topic, we shall defer it until after our discussion of evolutionary 



50 • THE EVIDENCE FOR EVOLUTION 

mechanisms. Our purpose now is to present the evidence that evolution has 
occurred in the past, and of this evidence, fossils constitute the major portion. 



SUMMARY < 



The fossil remains of animals and plants are widely dis- 
tributed over the earth. Absolute and relative dating methods 
show them to be of varying ages — some quite recent, others of 
great antiquity. These fossils constitute an actual record of the 
organisms that lived on the earth at different times in the past. 
An examination of this record shows that the kinds of living ani- 
mals and plants changed gradually with time. Thus, species ad- 
jacent in time are more alike than species separated by vast time 
spans, and the more recent the fossils, the more they tend to re- 
semble living species. The theory of evolution, of descent with 
modification, provides the most logical explanation for the fossil 
record. The living species of the past, forced to adapt to an ever- 
changing physical and biological environment, underwent gradual 
modifications through time. Many groups, unable to adapt, be- 
came extinct; others, more successful, survived and spread, only 
to be supplanted in turn by still better adapted types. These suc- 
cessful groups, however, did not arise de novo, but were de- 
scended from previously existing species of animals and plants. 

SUGGESTED READING 

Colbert, E. H., 1955. Evolution of the vertebrates. New York: Wiley. 

Flint, R. F., 1957. Glacial and Pleistocene geology. New York: Wiley. 

Moore, R. C, 1958. Introduction to historical geology, 2d ed. New York: McGraw- 
Hill. 

Newell, N. D., 1959. "The nature of the fossil record," Proc. Amer. Phil. Soc. 
103:264-285. 

Romer, A. S., 1945. Vertebrate paleontology, 2d ed. Chicago: University of Chicago 
Press. 

, 1958. The vertebrate story. Chicago: University of Chicago Press. 

Simpson, G. G., 1950. The meaning of evolution. New Haven: Yale University 
Press. (New York: Mentor Books, 1951.) 

, 1951. Horses. New York: Oxford University Press. 

, 1953. Life of the past. New Haven: Yale University Press. 

, 1953. The major features of evolution. New York: Columbia University 

Press. 

Stirton, R. A., 1959. Time, life, and man. The fossil record. New York: Wiley. 



CHAPTER 



The Origin of the Earth 

and of the Universe 



Once it is known that the first fossils are several hundred 
million or a few billion years old, the next question inevitably 
concerns the origin of life and, beyond that, the origin of the 
earth and of the universe itself. Though cosmogony is currently 
making great strides, the answers to these questions are more 
speculative than those about the less remote events detailed in 
the fossil record. Nevertheless, a brief review of current thought 
on these questions is certainly worthwhile, as long as it is realized 
that this sort of information has a different basis and hence is 
less reliable than the reconstruction of past events based on actual 
fossil remains. The theories in these areas are much more likely 
to change as new information becomes available. 

On the basis of narratives in the Old Testament, Arch- 
bishop Ussher in the seventeenth century calculated that the 
world was created in 4004 B.C. The delvers into such mysteries 
among the people of ancient India arrived at a date that would 
in 1962 make the world 1,972,949,063 years old. Modern esti- 
mates, which do not claim such precision, generally agree that the 
zero hour of the universe, as we know it, was a few billion years 
ago. 

Age of the Universe 

Science has used several approaches to estimate the age 
of the earth. One of these is a method that determines the age of 
the oceans. About 3 percent of sea water consists of dissolved salts. 



51 



52 • THE EVIDENCE FOR EVOLUTION 

These salts are constantly being leached from the rocks forming the earth's crust 
and are carried to the oceans by the rivers. The water evaporates from the surface 
of the oceans, falls on the land, and again flows to the sea in an ever-renewed 
cycle, but the salts remain in the sea, the salinity gradually increasing as time 
passes. Each year about 400 million tons of salt are added to the 40 X 10 15 tons 
already present in the seas. Simple division indicates that the process has lasted 
for at least 100 million years. However, since the rate of erosion is now un- 
usually high compared to other periods of geological time, because of the higher 
mountain ranges and man's activities, this estimate must be increased at least 20 
to 30 times, which leads to an age of 2 or 3 billion years. The very fact that the 
oceans are not saturated with salt indicates their limited existence. 

The age of the continents can be determined by estimating the age of the 
rocks composing them. The radioactive elements uranium and thorium are found 
in small quantities in many rocks, where both slowly decay into lead. Once the 
rock has solidified, the radiogenic lead cannot escape, but remains trapped in 
the rock with the original radioactive substances. The uranium/lead and 
thorium/lead ratios give a rather exact figure for the age of a given rock in 
much the same way an hour glass might if each grain as it fell were changed to 
lead. Different rocks give different ages, but the maximum estimate thus far is 
about 3 billion years. This value is fairly reliable, but must be regarded as the 
lower limit of the age of the earth, for the earth may well have been formed 
long before these rocks solidified into their present structure. Similar types of 
analyses have been run on meteorites in an effort to estimate the age of the solar 
system. The age of the meterorites was found to be on the order of 4.5 billion 
years; the earth, as a part of the solar system, must also be approximately of this 
age. Still another possible type of analysis is the determination of the age of the 
chemical elements themselves — that is, the matter that forms the solar system. 
These elements must have a finite age; otherwise, by now the radioactive ele- 
ments would have disintegrated and disappeared. Estimates of their age range 
up to 6 billion years. 

Astronomers have tackled the age of the universe in several ways. One 
method is to study stellar velocities within the Milky Way, the galaxy of which 
we are a part. When such a system has existed for a long time, the stellar 
velocities are expected to approach a limiting distribution with an equal partition 
of energy among all the stars. However, this distribution has not yet been real- 
ized, and the calculations indicate that the system has existed only a few billion 
years. 

A second method is based on the rate at which a star burns up. Stars 
obtain their energy from the nuclear transformation of hydrogen into helium in 
their hot centers. Thus, the life span of a star is determined by its brightness or 
rate of burning and by its original hydrogen content. Larger stars burn out faster 
than the smaller ones such as the sun. The large stars that must have been 



ORIGIN OF EARTH AND UNIVERSE • 53 

formed several billion years ago are now in their death throes, pulsating and 
exploding, but the smaller sun has at least 5 billion years to go before reaching 
this stage. 

A third method of estimating the age of the universe is based on what 
is called the "red shift." The Milky Way, containing billions of stars, is just one 
of about a billion such stellar systems or galaxies within the range of the 200- 
inch telescope at Mount Palomar, California. A peculiar feature about the dis- 
tant galaxies is that the light from them, although similar to that from nearer 
ones, shows a peculiar shift of the spectral lines toward the red end of the 
spectrum. A simple physical explanation for this shift is that the galaxies are 
receding at high speeds, and hence the universe is expanding. The effect is 
similar to the apparent change in pitch of a train whistle as it approaches and 
then recedes from a crossing. Since this phenomenon, known as the Doppler 
effect, has also been reported in the rather new field of radio astronomy, both 
light and radio waves appear to be similarly affected. 

This discovery led to still another method of estimating the age of the 
universe, on the assumption that the universe as we know it today arose as the 
result of the differentiation of some sort of rapidly expanding primordial matter. 
A date for the beginning of this expansion can be obtained by dividing the 
average distance between neighboring galaxies by the velocity of their recession. 
The original estimate by this method, 1.8 billion years, presented a puzzle be- 
cause the geological estimates already were much greater than this. Recently, 
however, corrections in this method have led to estimates for the age of the uni- 
verse as high as 7 to 10 billion years. Although some differences exist in the 
various estimates, they are not too important for our purposes. Trie age of the 
universe, as derived from several independent estimates, seems to be about 
5 billion years or more. 

Nature of the Universe 

The findings of the astronomers have led modern cosmologists to two 
quite different conceptions of the nature of the universe. One is that of an 
evolving universe, the other a steady-state universe. Under the evolutionary 
theory the expansion indicated by the red shift is interpreted to mean that the 
universe started off with a "big bang." The matter within the universe was 
squeezed together so tightly and at such high temperature and density that it 
consisted only of protons, neutrons, and electrons, which did not form any 
larger elements. When, because of expansion, the temperature dropped, the 
neutrons started to decay to protons, and the neutrons and protons started to 
form aggregations of atomic nuclei. The rate of expansion determined the types 
of atoms formed. Physicists have calculated that the "cooking period" could not 
have lasted more than half an hour. If it had been less (a rapid expansion), the 



54 • THE EVIDENCE FOR EVOLUTION 




Fig. 5-1. The origin and evolution of the earth. 



universe would contain mostly hydrogen; if longer, heavy elements would 
predominate. 

For the next 250 million years, radiant energy was predominant over 
matter (the two being interconvertible with the now famous relationship E = 
mc 2 of Einstein). As expansion continued, the radiant energy was used to do the 
work of expansion, and matter became more prominent. At 250 million years 
the mass density of matter and radiation became equal. Prior to that time matter 
could be thought of as being "dissolved" in thermal radiation like salt in water. 

At this time matter and gravitation became predominant, and the dif- 
ferentiation of the previously homogeneous system began. Gas balls were formed 



ORIGIN OF EARTH AND UNIVERSE • 55 

of the mass of a galaxy (about 40,000 light years radius and 200 million times 
the mass of the sun). These dark gas clouds next differentiated or condensed 
into stellar gas balls that contracted rapidly. The compression raised the tempera- 
ture to 20,000,000 degrees, the threshold of nuclear reactions, and the stars 
began to shine. As most of the material fell toward the center of a star, the 
planets were formed from what was left behind. Colliding dust particles formed 
larger chunks of matter that swept through space, growing larger all the time. 
The process of star condensation and planet formation must have taken a few 
hundred million years. Since the moon is gradually moving further away from 
the earth, it appears that several billion years ago earth and moon formed a 
single mass, from which the moon has broken away. This conception of the 
universe extends the theory of evolution to the universe itself. 

The steady-state theory, on the other hand, suggests that the universe is 
infinite in both space and time, that the density of its matter remains constant, 
and that new matter is constantly being created throughout space at a rate just 
compensating for the thinning of matter by expansion, with new galaxies con- 
stantly being formed. 

The major difficulty with the theory of an initial 30-minute "cooking 
period" is that there are no stable atoms of mass 5 or mass 8, and therefore the 
build-up of the heavier elements by neutron capture could not get past helium 4. 
This shortcoming in the theory has led Gamow, one of its proponents, to agree 
recently that the bulk of the heavy elements may have been formed later in the 
hot core of stars. 

Two recent tentative advances have lent still further support to the 
concept of an evolving universe. The steady-state hypothesis postulates that the 
density of matter in space remains contant. The density of radio stars, however, 
increases with distance. Since most radio stars are apparently due to collisions 
between galaxies, this latter finding indicates that galactic crack-ups were more 
common billions of years ago when these signals started on their way than they 
are today. Since the evolutionary theory postulates a denser universe then, with 
collisions between galaxies therefore more probable, this discovery, if confirmed, 
gives strong support to the theory. 

In studying the red shift, distance is measured in light years rather than 
miles. The speed of light is 186,000 miles per second, yet some galaxies are 
millions of light years away. In viewing these far-distant galaxies, we are looking 
not only over great distances but also backward in time. A study of clusters of 
galaxies about a billion light years away has shown that a billion years ago the 
universe was expanding faster than it is today. If the rate of expansion is slow- 
ing down, then we must live in an evolving rather than a steady-state universe. 
Furthermore, the slowing down suggests that eventually expansion will stop and 
contraction will begin, ultimately reaching the superdense condition that existed 
some 5 or more billion years ago. The concept of a pulsating universe is thus 



56 • THE EVIDENCE FOR EVOLUTION 

further strengthened. As for the question of the structure of the universe prior 
to the colossal explosion that started it, it seems likely to remain inscrutable, for 
whatever previous structure existed was lost in the dense mass of energy, elec- 
trons, protons, and neutrons that gave rise to our present universe. 



SUMMARY < 



Our knowledge of the origin of the earth and of the 
universe is neither as specific nor as detailed as our knowledge of 
the evolution of plants and animals derived from the study of 
fossils. Nevertheless, progress in the fields of physics, chemistry, 
and astronomy has made it possible to attack this question on a 
rational, scientific basis. The results of these studies indicate that 
the earth was formed several billion years ago and that the age 
of the universe as we know it is approximately 5 to 10 billion 
years. Although the alternative hypothesis of a steady-state uni- 
verse has been advanced, there is considerable evidence to indicate 
that the universe itself is an evolving system, changing through 
time. 

SUGGESTED READING 

Bondi, H., 1952. Cosmology. New York: Cambridge University Press. 
Brown, H., 1957. "The age of the solar system," Sci. Amer., 196(4) :80-95. 
Gamow, G., 1951. "The origin and evolution of the universe," Amer. Sci., 39/393- 

406. 

, 1952. The creation of the universe. New York: Viking. 

Hoyle, F., 1955. Frontiers of astronomy. New York: Harper. (New York: Mentor 

Books, 1957.) 
Pfeiffer, J., 1956. The changing universe. New York: Random House. 
Robertson, H. P. et al., 1956. "The universe," Sci. Amer., 195(3) :72-236. 
Russell, B., 1958. The ABC of relativity. London: Allen and Unwin. (New York: 

Mentor Books, 1959.) 
Schwarzschild, M., 1958. Structure and evolution of the stars. Princeton: Princeton 

University Press. 



CHAPTER 



The Origin of Life 



Since man tends to seek final answers to all major ques- 
tions, it is not surprising to find some sort of explanation for the 
origin of the world, of life, and of man in practically every 
human culture. These beliefs fall into the realm of myth or super- 
stition in many cases or they may be a part of the religion of the 
society. So intriguing a question as the origin of life has a number 
of theories associated with it, most of which can be grouped into 
a few major categories. One category involves a belief in the 
creation of life by a supernatural creator, an explanation that is 
outside the realm of science and therefore not open to scientific 
study. Another category, however — that of spontaneous generation 
— does admit of such investigation. 

Spontaneous Generation 

For centuries, the problem of the origin of life did not 
loom large in men's minds, for it was common knowledge that 
life was arising de novo all around them all the time. As if by 
magic, worms appeared in their rain barrels, maggots in their 
meat, and mice in their rag bags; hence the spontaneous genera- 
tion of worms, maggots, and mice, where none had previously 
existed, was a fact easily demonstrated from everyday experience. 
Among the Greeks, Thales, Anaximander, Xenophanes, and Aris- 
totle all believed in some form of spontaneous generation. Even 
such scientists as Harvey, Newton, Descartes, and Paracelsus cen- 
turies later believed in it, and van Helmont, who did notable 



57 



58 • THE EVIDENCE FOR EVOLUTION 

early work on plant nutrition, left a recipe for the spontaneous generation of 
mice — namely, a sweaty shirt plus some wheat germ. 

Some of the fables are so fantastic that it is difficult to conceive how 
they originated. For example, according to the goose tree legend of the Middle 
Ages, geese were derived from barnacles, which in turn were formed in the 
fruits of trees. Since geese were thus obviously of vegetable origin, for centuries 




Fig. 6-1. The goose tree legend. 



they were an acceptable meat substitute during Lent. This belief was periodically 
reinforced by careful observations, often accompanied by imaginative drawings 
(see Fig. 6-1), and it persisted even to the beginning of the seventeenth 
century. One possible explanation for the origin of the legend is the coincidence 
of the time of attachment of the marine barnacles in the northern British Isles 
with the arrival of migrating young geese from the Arctic. These barnacles attach 
to a variety of things in the water, including fallen trees or branches, and this 
fact may have been the basis for the strange juxtaposition of beliefs. 

Not until the seventeenth century were the first real doubts cast on the 



THE ORIGIN OF LIFE • 59 

theory of spontaneous generation. The experiments of an Italian, Francisco Redi, 
showed that meat, covered with a cloth so that flies could not lay their eggs on it, 
never developed maggots. The idea nevertheless persisted, especially in relation 
to microorganisms. A century later, Spallanzani sealed some broth in a flask, 
boiled it, and showed that no microorganisms then developed and hence no 
spoiling occurred for an indefinite period. Needham, however, objected that the 
broth and particularly the air in the flask were changed by the boiling so that 
they would not support life. Breaking the seal on the flask, Spallanzani showed 
that the broth would still support life, but he failed to answer the criticism con- 
cerning the air. Hence, belief in spontaneous generation persisted not only 
among people generally but among biologists until less than 100 years ago. The 
experiments of Pasteur finally ended the argument, and the axiom became Omne 
vivum e vivo for all beginning biology students. Pasteur's proof was a simple 
modification of Spallanzani's experiment. Rather than sealing the flask, he drew 
the neck out into a thin undulating tube, open to the air. After boiling, the 
broth remained sterile because dust and bacteria and mold spores were trapped 
in the neck of the flask even though the air molecules had free passage. After 
Pasteur had completed his painstaking series of experiments, no satisfactory 
explanation for the origin of life remained. Special creation was not a scientific 
explanation, and spontaneous generation had been shown not to occur. However, 
it might be noted at this point that negative proof can never be regarded as final. 

An interesting twist in the theories was the concept that nonliving sub- 
stances came from living things rather than vice versa, a form of vitalism based 
on the idea that life itself is eternal. Another more or less related theory involves 
cosmozoa, living particles dispersed throughout the cosmos that take up their 
abode and evolve whenever conditions become suitable. Various methods of 
their transmission through space have been suggested; Richter proposed floating 
particles, von Helmholtz that they arrived via meteorites, and Arrhenius that 
they were propelled by the pressure from light rays. There is, however, no evi- 
dence supporting the existence of cosmozoa; indeed, the known effects of tem- 
perature, ultraviolet rays, and radiation on living organisms make the theory 
very improbable. Even if it were correct, the question of the origin of life is not 
answered, but is simply removed to some more inaccessible part of the universe 
unless it is assumed, as has been done, that cosmozoa are eternal. The theory, in 
sum, is far from adequate. 

In recent years, a new attack has been made on the problem, and the 
result has been, interestingly enough, a new version of spontaneous generation. 
The theory proposes that life originated on earth in the past when conditions 
were different from those of the present, and was preceded by a gradual chem- 
ical evolution that ultimately gave rise to self-duplicating molecules. Pasteur's 
experiments did not eliminate this possibility, for they demonstrated only that 
life did not originate spontaneously under his experimental conditions. 



60 • THE EVIDENCE FOR EVOLUTION 
The Composition of Living Things 

In order to discuss the conditions under which life might have origi- 
nated in the past, we must have some idea of the nature of living things. They 
are composed of water, inorganic salts, and carbon compounds — organic sub- 
stances known as carbohydrates, fats, proteins, and nucleic acids. The nucleic 
acids in combination with protein form the hereditary material; proteins form 
the structure of the organism; and the fats and the carbohydrates such as starch, 
glycogen, and the sugars are primarily a source of energy for cellular work. 
These compounds are highly organized into a smoothly functioning whole in the 
living organism. Thermodynamically, a living animal is a very improbable 
structure. The complex molecules are built up from relatively few elements, 
actually only 20 or so out of the 95 available on the earth. Carbohydrates and 
fats are formed from carbon (C), oxygen (O), and hydrogen (H) alone, and 
these three elements and the nitrogen (N) essential to protein formation form 
99 percent of living protoplasm. Sulfur and phosphorus are two other important 
elements, found in proteins, for example. The inorganic salts are formed prima- 
rily from sodium, potassium, calcium, magnesium, and chlorine; traces of iron, 
copper, manganese, zinc, cobalt, nickel, iodine, vanadium, fluorine, boron, alumi- 
num, and bromine have been found in various species of plants or animals. 

The availability of the elements does not determine their utilization in 
living organisms, for some very common elements in the earth's crust are either 
absent or present in very low concentrations in organisms. Hence, some sort of 
selective process must be involved. The unique feature about hydrogen, oxygen, 
nitrogen, and carbon is that they are the smallest four atoms that can become 
stable by gaining 1, 2, 3, and 4 electrons respectively in their outer shell of 
electrons. They share electrons with other atoms to form chemical bonds that 
lead to molecule formation. Phosphorus and sulfur are in the same relative posi- 
tion in the periodic table as nitrogen and oxygen, but they are one group higher. 
The lightest elements (C, H, O, N) are the only ones that regularly share two 
or even three pairs of electrons with other atoms and hence permit the building 
up of chains of atoms. Silicon is chemically similar to carbon and much more 
available in the earth's crust, but, lacking this electron-sharing ability, is seldom 
found in living organisms. The trace elements such as the iron in hemoglobin 
or the magnesium in chlorophyll are complex formers, holding together big 
molecules. 

Water, which is a major component of organisms, is a unique sub- 
stance. It is the best solvent known, and has a long liquid range — that is, a high 
boiling point and a low freezing point. It promotes the ionization of salts 
through its high dielectric constant, and it expands from 4° C down to 0° C, its 
freezing point. 



THE ORIGIN OF LIFE • 61 

Formation of Organic Compounds 

Our previous discussion of the formation of the present universe indi- 
cated that the elements were not likely to be bound together in large molecules; 
in other words, organic compounds such as carbohydrates, fats, and proteins 
were not present on the earth during its formative period. Life could not have 
originated on the earth until the earth had assumed more or less its present form; 
thus, before we can talk of the origin of life, we must discover what conditions 
prevailed on the earth several billion years ago and what means were available 
to cause the synthesis of the more complex compounds from the very simple ones 
that existed then. Unfortunately, these questions are not easy to answer. For 
example, it is not certainly known whether the earth's atmosphere then contained 
free oxygen; prevailing opinion is that no free oxygen was present and that the 
atmosphere was reducing in character. However, several mechanisms have now 
been demonstrated experimentally by which more or less complex organic mole- 
cules can be obtained from simple carbon compounds such as formic acid or 
methane and nitrogenous substances such as ammonia or nitric acid or nitrates. 
Shown below are some of the structural formulas of compounds mentioned in 
the text. 

H H H O O 



H— O N— H 

I 
H 



water ammonia 

O 

II 

C— OH 

I 
C— OH 



H— C— H 

I 
H 

methane 



o=c=o 



N— OH H— C— OH 



H 

I 
H— C— H 

I 
C— OH 



O 

oxalic 
acid 



O 

acetic 
acid 



carbon 
dioxide 



O 

II 

C— OH 

H— C— H 

I 
H— C— H 

I 
C— OH 



O 



nitric 
acid 
O 

II 

C— OH 

I 
H— C— OH 

I 
H— C— H 

I 
C— OH 



formic 
acid 



H— N— H 
I 

c=o 

I 

H— N— H 

urea 



Ca 



calcium 
carbide 



H-0=C— H 

acetylene 



O 

succinic 
acid 

H O 



O 

malic 
acid 



H— C— C— OH 

I 
N— H 

I 
H 

glycine 



H- 



H H O 




—OH 



62 • THE EVIDENCE FOR EVOLUTION 

At present, living things directly or indirectly get their free energy 
from sunlight by means of the photosynthetic process in green plants. Before 
the evolution of photosynthesis, other energy sources had to be used because 
simple molecules such as CH 4 , H 2 0, NH 3 and so on do not absorb light in the 
visible spectrum. Only after the appearance of compounds like the porphyrins 
(for example, chlorophyll) or other pigments did absorption in the visible spec- 
trum become possible. The energy sources that could have made significant con- 
tributions to the early synthesis of organic compounds appear to have been 
primarily ultraviolet light and electric discharges such as lightning. The possible 
contributions of energy from cosmic rays, radioactivity, or volcanoes seem to have 
been very slight. Although thermal synthesis of organic compounds has been 
suggested, its significance has been questioned. The surface of the primitive earth 
is thought to have been cool, as the result of its formation from the condensa- 
tion of a cold cloud of cosmic dust, and therefore unfavorable to this type of 
synthesis. 

A number of experiments to demonstrate possible methods for the 
synthesis of organic compounds prior to the existence of living organisms have 
been performed. One type of experiment involved the illumination of aqueous 
solutions of these simple compounds with ultraviolet light; the result was forma- 
tion of amino acids and heterocyclic or ring compounds. In another experiment, 
water vapor, ammonia, methane, and hydrogen, substances all thought to have 
been present in the primitive reduced atmosphere, were passed over an electric 
spark to simulate the effects of electric discharges in the upper atmosphere. The 
amino acids, glycine and alanine, plus several others were recovered after a week. 
Still another method was suggested by the Russian biochemist, Oparin, who 
initiated the recent discussions on chemical evolution with his book The Origin 
of Life published in 1936. He suggested that the earth, cooling from a hot 
miasma, had its carbon primarily in the form of metallic carbides, which, on 
coming in contact with water, formed the hydrocarbon, acetylene. The acetylene 
then could polymerize under the influence of catalysts to form the longer carbon 
chain molecules. Furthermore, the thermal production and conversion of amino 
acids from malic acid and urea has also been demonstrated. Finally, a fifth 
method to be tested experimentally was the effect of very high energy radiation 
such as that from cosmic rays or from radioactive minerals. In this manner solu- 
tions of carbon dioxide and water have been irradiated to form formic acid; the 
formic acid has then produced the 2 -carbon compounds, oxalic acid and acetic 
acid, and even the 4-carbon compound, succinic acid, but all in very low con- 
centrations. Just which conditions prevailed and which mechanisms were impor- 
tant billions of years ago cannot yet be stated with certainty. The important 
point is that several mechanisms have been demonstrated by which organic com- 
pounds, those with carbon-carbon or carbon-hydrogen bonds, can be formed 
without the mediation of living organisms. 

Granted, then, that organic compounds could have been formed; the 



THE ORIGIN OF LIFE • 63 

next logical question concerns their stability. Today, organic substances are 
rapidly destroyed, primarily by decay or oxidation. Decay is due to the activities 
of living microorganisms, but since no life existed at the time we are discussing, 
the organic compounds were not then subject to this kind of decomposition. 
Furthermore, since it is generally thought that free oxygen was virtually absent 
from the earth's early atmosphere, organic matter was not subject to oxidation 
either, and hence could accumulate on the earth's surface. A further point of 
interest is the belief that carbon dioxide, like oxygen, was essentially absent from 
the early atmosphere though now both are common in the air. The conclusion 
to be drawn is that both oxygen and carbon dioxide are present in the atmos- 
phere because of the activities of living organisms; oxygen because of its release 
during photosynthesis by plants, carbon dioxide due to the respiration or meta- 
bolic activity of almost all living things. 

Although the early organic compounds were not subject to decay or 
oxidation, they were not entirely stable. Just as "spontaneous" formation of 
organic matter was undoubtedly possible, so was "spontaneous" decomposition, 
since chemical reactions are reversible, and some sort of equilibrium between 
synthesis and decomposition is achieved. Furthermore, because of the energy 
relations between the various compounds, the equilibrium point is usually far on 
the side of decomposition. Thus, although amino acids have a certain probability 
of uniting to form polypeptides or even proteins, the probability that a protein 
or polypeptide will break up into its constituent amino acids is far greater. 

At this point in our chronology we have a more or less random assort- 
ment of simple, relatively stable organic molecules, such as amino acids, in the 
form of a dilute aqueous solution — a rather thin broth — still a far cry from even 
the simplest of living organisms. Present-day organisms can only maintain them- 
selves and grow by a constant expenditure of energy drawn from their environ- 
ments. A living organism is, in a sense, a chemical machine, which, because of 
its organization and metabolic activity, is able to take up materials and energy 
from the environment and incorporate them iryorder to survive, grow, and repro- 
duce itself. The next question is the crux of trie problem of the origin of life: 
How, from the dilute broth of organic compounds, did higher types of organiza- 
tion arise, persist, and ultimately lead to self duplicating entities ? Unfortunately, 
our knowledge here is only a beginning toward complete understanding. How- 
ever, various suggestions have been made as to ways in which large molecules, 
once formed, are kept from breaking up. If the molecules are removed from 
solution by precipitation, they no longer are so apt to disintegrate. Similarly, by 
becoming attached to other molecules, they are "trapped" in their more complex 
form. In this fashion, molecular aggregates of considerable complexity could 
have been built up in a stepwise fashion. Furthermore, the orderly propensities 
of matter — their tendency toward forming crystals, for example — could also 
have played a role in bringing structure to the random assortment of substances. 
This order is inherent in the molecules. Muscle or cartilage fibers, after being 



64 • THE EVIDENCE FOR EVOLUTION 

dissolved, will return, on precipitation, to their original molecular patterns. 
Proteins are composed of long chains of amino acids connected by peptide 
linkages (that is, a bond formed between the carboxyl group ( — COOH) of 
one amino acid and the amino group ( — NH 2 ) of the next with the elimination 
of H 2 0). Since these bonds are broken or hydrolyzed in water, it has also been 
proposed that the long polypeptide chains were first formed by polymerization 
in, for example, a dried-up pool in the absence of water rather than in the 
primordial "soup." 

Perhaps the most characteristic trait of living things is their ability to 
reproduce their own kind. It is at this point that we must begin to think in terms 
of chemical evolution governed by a selective process akin to natural selection. 
Some chemical compounds are catalysts for their own formation; in a more or 
less random group of molecules or aggregates, an autocatalytic compound will 
have a selective advantage over the others, for it will tend to transform the 
others into itself or, in the competition for substrate, it will win out as each new 
unit in turn catalyzes the formation of others like itself. Furthermore, the more 
efficient autocatalysts will win out in competition with the less efficient types so 
that in time very efficient self-duplicating systems will arise. If these molecular 
aggregates become unstable when they exceed a certain size, they will break up, 
and the cycle of self-duplication will then start anew. 

Sources of Energy and Food 

Finally, we should consider the ways in which living organisms get the 
energy they need to continue to exist. This energy must be externally derived 
by the organism. Not only must the energy be obtained, but it must be available 
in such a form that the organism can make use of it. Today living things obtain 
their energy by means of coupled reactions in which one reaction gives off 
energy to another that absorbs it. Probably the most important of such coupled 
reactions in present organisms is oxidative phosphorylation, by means of which 
the energy from burning (or oxidizing) sugar is made available to do cellular 
work rather than being lost as heat. Instead of being released in one large burst, 
the oxidation is stepwise, and at each step a little parcel of energy is tied up as 
chemical energy in a molecule known as adenosine triphosphate (ATP). The 
formation of a single peptide linkage in a protein requires a small amount of 
free energy, energy that can be obtained through a coupled reaction with an 
ATP molecule. The energy exchanges involving ATP are useful not only in 
protein synthesis but also in muscle contraction and in a variety of other ways in 
the cell. The unique feature of the ATP molecule is that two of its three phos- 
phate groups are linked together by what are known as "energy-rich" or "high- 
energy" phosphate bonds. The significant property of these phosphate groups is 
that in transfer to another compound they carry with them a certain amount of 
free energy, and in this way supply the energy needed to do cellular work at the 



THE ORIGIN OF LIFE • 65 

time, in the place, and in the amounts needed. The efficient energy-coupling 
systems involving ATP and catalyzed by enzymes undoubtedly are the product of 
the evolutionary process and are probably derived from simpler, less efficient 
systems in the past. 

In addition to energy, the living organism if it is to live, grow, and 
reproduce requires food. The source of food for primitive organisms, formed 
under the conditions described previously, must have been the other organic 
molecules in the aqueous broth. Since oxygen was absent, the only process avail- 
able was fermentation, by which energy is obtained from the breakage and re- 
arrangement of organic compounds in the absence of oxygen. A typical fer- 
mentation is that of sugar by yeast to yield alcohol, carbon dioxide, and energy. 

C 6 H 12 6 -> 2C0 2 -f 2C2H5OH + energy 

glucose carbon ethyl 

dioxide alcohol 

The C0 2 and alcohol are waste products in the cell and must be eliminated. 
Fermentation is a destructive process, however, and the exhaustion of the avail- 
able organic compounds would have led to a cessation of life. 

The next step must have been the evolutionary invention of photo- 
synthesis, made possible by the quantities of C0 2 released by fermentation. Thus 
it became possible for living organisms to synthesize their own organic mole- 
cules, using the energy from the sun. The equation 

6CO2 + 6H 2 ■££> C 6 H 12 6 + 60 2 

light 

carbon water glucose oxygen 

dioxide 

shows the synthesis of sugar; nitrogen was available from inorganic nitrates or 
ammonia, and therefore all of the necessary organic compounds could be synthe- 
sized. Living things now were no longer dependent on the accumulated organic 
matter from the nonliving era, but could synthesize needed materials by photo- 
synthesis and obtain necessary energy by fermentation. 

The oxygen production by photosynthesis provided a much more effi- 
cient source of energy, however. The waste products of fermentation — alcohol, 
lactic acid, formic acid, etc. — are poisonous, and the energy yield is low. The 
process of respiration, or the combination with oxygen, is much more efficient, 
for the energy produced is about 35 times as great for the same amount of sugar 
consumed. All possible energy is extracted; thus a maximum amount of energy 
is obtained from a minimum amount of material. Furthermore, the waste prod- 
ucts, carbon dioxide and water, are harmless and easily disposed of. The equation 
for respiration is 

C 6 H 12 6 + 60 2 -> 6C0 2 + 6H 2 + energy 

glucose oxygen carbon water 

dioxide 



66 • THE EVIDENCE FOR EVOLUTION 

The processes of photosynthesis and respiration have made life, as we 
know it today, possible. In tending to pride ourselves on our progress and on 
our control over the environment, we sometimes overlook man's complete de- 
pendence on energy from the sun for his very existence. Since fermenting organ- 
isms have never evolved to a very high degree of organization and complexity, 
it seems reasonable to suppose that only with the origin of respiration did the 
evolution of more complex organisms, including man, become possible. 

Therefore, the current hypotheses of the origin of life envision initially 
the random formation of more or less complex organic compounds from the 
simpler molecules present in what was probably a reducing atmosphere. Auto- 
catalytic molecules, having a selective advantage over the other types, tended to 
increase in frequency. At what point one should stop speaking of molecules and 
start referring to living organisms is rather difficult to say. However, since a self- 
duplicating system capable of mutation is frequently regarded as the fundamental 
criterion for life, by this standard we are already discussing living systems. The 
original organisms were heterotrophic, obtaining their essential constituents from 
the environment rather than synthesizing them from carbon dioxide and water. 
Evolution of additional enzyme systems as a result of the selective process then 
led to autotrophic organisms capable of carrying out increasingly complex and 
efficient syntheses from very simple precursor substances. The exact steps by 
which cellular life as we know it today arose through the process of chemical 
evolution cannot be stated with certainty. Nevertheless, some of the basic ques- 
tions involve the origin of protein synthesis, of deoxyribonucleic acid as the 
genetic material, of high-energy organic phosphates such as ATP, of catalytic 
compounds or enzymes, particularly the porphyrins, and the origin of cell struc- 
ture. Although answers to these questions are at present rather speculative, active 
research in this field is in progress, and at a recent symposium on evolution, a 
panel of experts was unanimous in agreeing that the synthesis of life was both 
conceivable and possible in the not too distant future. 

Hence, the origin of life cannot be regarded as a mysterious, unique 
process but, rather, one that was practically inevitable and, moreover, will occur 
whenever and wherever similar conditions exist. Since billions of planets like the 
earth are scattered throughout the universe, it is conceivable that life exists in 
many more places than the earth. The astronomer Harlow Shapley has estimated 
very conservatively that there are approximately 100,000,000 planets in the uni- 
verse capable of supporting life similar to that on the earth. None of the details 
of this account can be taken too seriously or as finally established, and to some 
people it may seem no more than a modern fable of the origin of life, com- 
parable to those of the ancients and with a similar purpose. Nevertheless, there 
is sufficient evidence to consider it a reasonable hypothesis worthy of further 
study. 



THE ORIGIN OF LIFE • 67 

► SUMMARY 



Again, as with the origin of the universe, recent scientific 
advances have made it possible to attempt to answer the question 
of the origin of life on a rational basis and even to tackle it ex- 
perimentally. Present theories recognize that life arose when the 
physical conditions on the earth were quite different from those at 
present. A long period of chemical evolution is thought to have 
preceded the origin of the first self-duplicating particles that 
could be called living. The earliest forms of life are thought to 
have been saprophytic, deriving energy from the fermentation of 
organic compounds in the environment. Only later did living 
cells evolve the ability to synthesize complex molecules from 
simple precursors, a trend that culminated in the evolutionary in- 
vention of photosynthesis. Respiration, a far more efficient process 
of energy extraction than fermentation, only became possible after 
the oxygen in the atmosphere increased as a result of photo- 
synthesis. 

SUGGESTED READING 

Blum, H. F., 1955. Time's arrow and evolution, 2d ed. Princeton: Princeton Univer- 
sity Press. 

Calvin, M., 1956. "Chemical evolution and the origin of life," Amer. Set., 44:248- 
263. 

, 1959. "Evolution of enzymes and the photosynthetic apparatus," Science, 

230:1170-1174. 

, 1959. "Round trip from space," Evolution, 23:362-377. 

Fox, S. W., 1956. "Evolution of protein molecules and thermal synthesis of bio- 
chemical substances," Amer. Sci., 44:347-359. 

Gaffron, H., I960. "The origin of life," Evolution after Darwin, Vol. I, The Evolu- 
tion of life. S. Tax, ed. Chicago: University of Chicago Press. 

Miller, S. L., 1953. "A production of amino acids under possible primitive earth 
conditions," Science, 2 27:528-529. 

, and H. C. Urey, 1959. "Organic compound synthesis on the primitive earth," 

Science, 230:245-251. 

Oparin, A. I., 1957. The origin of life on the earth, 3d ed. New York: Academic 
Press. 

, et al., eds., 1959. The origin of life on the earth. Pergamon Press. Reports 

of the Moscow Symposium on the origin of life. August 1957. 

Pringle, J. W. S., 1953. "The origin of life," Symposium Soc. Exp. Biol., 7 (Evolu- 
tion): 1-21. New York: Academic Press. 

Wald, G., 1954. "The origin of life," Sci. Amer., 292(2) :44-53. 



CHAPTER 



7 

Geographical Distribution 



The physical evidence for evolution consists of living 
organisms and the remains of organisms that have lived in the 
past. Although the fossil record presents concrete evidence that 
species differing from all living species lived long ago, it is often 
sketchy or incomplete on critical points. If the record were com- 
plete, we would have before us the complete phylogeny of all 
living things and there would be no need to seek further infor- 
mation by more indirect methods. However, because of the 
paucity of the fossil record, it has been necessary to turn to living 
organisms to plot more fully the course of past evolution. The 
study of the present geographical distribution of animals and 
plants has lent considerable support to the theory of evolution. 

In our discussion of adaptation we noted that organisms 
are adapted to their environments. It is now necessary to analyze 
this situation still further. Within a given geographical area, the 
environment is not uniform; in other words, a great variety of 
different types of habitat exist. In the state of Minnesota, for ex- 
ample, three major types of terrestrial habitat can be recognized: 
the deciduous forest in the southeast, the coniferous forest to the 
north, and the prairie in the west and southwest. If the variety of 
fresh-water habitats to be found in the thousands of lakes, and in 
the streams, swamps, bogs, and rivers is included, the range of 
possible habitats becomes even wider. Yet each species has its own 
ecological niche, its own unique requirements of the environment; 
where these are not met, that species is not to be found. To use 
a painfully obvious example, the fish in Minnesota are confined 

68 



GEOGRAPHICAL DISTRIBUTION • 69 

to the water. Much more subtle differences than that between fresh water and dry 
land may determine whether a species will be found in a particular spot; thus, 
within a given area such as Minnesota, the ecological conditions may vary widely, 
and the species present will vary also in accordance with the changes in ecolog- 
ical factors. Though no physical barrier exists, the animals and plants to be found 
in the deciduous forest areas of southeastern Minnesota are distinctly different 
from the animals and plants to be found in the coniferous forests to the north, 
and surprisingly few species are common to both areas. 

But, and this is a very important "but," there is another aspect to dis- 
tribution, which can be most readily outlined by quoting from Darwin. 

Neither the similarity nor the dissimilarity of the inhabitants of various 
regions can be wholly accounted for by climatal and other physical conditions .... 
There is hardly a climate or condition in the Old World which cannot be paralleled 
in the New — at least as closely as the same species generally require .... Not- 
withstanding this general parallelism in the conditions of the Old and New Worlds, 
how widely different are their living productions. 

For example, the climates of parts of Australia, South Africa, and 
western South America are very much the same, but the fauna and flora in each 
region are strikingly different. In South America, on the other hand, the species 
south of 35° latitude and those north of 25° latitude are clearly quite similar, 
although they live under markedly different climatic conditions. 



Biogeographical Realms 

Because species living in the same region tend to resemble each other 
despite considerable differences in climate and habitat, it has been possible to 
delimit biogeographical realms, within which the existing groups of animals and 
plants show many similarities. These realms, shown in Fig. 7-1, are the 

1. Nearctic — North America down into the Mexican plateau in central Mexico. 

2. Palearctic — Asia north of the Himalayas, Europe, and Africa north of the 

Sahara Desert. Since the species of the Nearctic and Palearctic regions 
are much alike in many respects, these two regions are sometimes 
grouped together as the Holarctic. 

3. Neotropical — Central and South America. 

4. Ethiopian — Africa south of the Sahara. 

5. Oriental — Asia south of the Himalayas. 

6. Australian. 

Though the absence of a species because of an unsuitable environment 
is easy to appreciate, its absence when the environment is favorable poses other 
questions. There is little doubt that many species can survive and even thrive in 



70 • THE EVIDENCE FOR EVOLUTION 

regions other than the one in which they normally occur. The rapid increase and 
spread across the United States of the English sparrow and the starling intro- 
duced from Europe within the past century is a case in point. Further examples 
are the depredations of the Japanese beetle and the gypsy moth, two other species 
recently introduced into the United States. Many of our common roadside weeds 
and flowers also had their origin in Europe, but were brought here with seeds or 
escaped from gardens. The phenomenal increase in the number of rabbits in 
Australia, where they have become a serious pest in the absence of the predators 
found in their usual range, is striking evidence that ecological factors alone do 
not determine the distribution and numbers of animals. 




.^^^lEARCTIC n 



ETHIOPIAN /ORIENTAL' 



AUSTRALIAN/ 




Fig. 7-1. The biogeographical realms. 

Table 7-1 shows the distribution of some significant groups of mam- 
mals; a few comments may help emphasize some of its important aspects. The 
similarities between the Nearctic and Palearctic are quite obvious. The single 
metatherian or marsupial in the Nearctic is the opossum, and the edentate is the 
armadillo, both of which appear to have spread north from South America. The 
few primates of the Palearctic are found on the fringes of the Ethiopian and 
Oriental realms. Although no members of the camel group now exist in the 
Nearctic, large numbers of fossils indicate their presence in the past. Not only 
the same major groups but closely similar species within these groups are to be 
found in the Nearctic and Palearctic. 

The Neotropical realm is a curious mixture of "modern" and "primi- 
tive" mammals. The edentates are also characteristic and quite numerous. 



GEOGRAPHICAL DISTRIBUTION • 71 

The Ethiopian or African region has the richest mammalian fauna but 
lacks completely the monotremes and marsupials. The hoofed mammals or 
ungulates are a large and important group with many representatives of both 
the Perissodactyla (odd-toed) and Artiodactyla (even-toed) orders. There are 
many rodents, carnivores, insectivores, and primates. 

In the Oriental region, similarities to both the Ethiopian and Palearctic 
realms can be seen. For example, elephants, rhinoceroses, and antelope are com- 
mon to the Ethiopian and Oriental; deer (Cervidae), and sheep and goats to the 
Palearctic and Oriental (and Nearctic). 

TABLE 7- 1 
Distribution of Certain Mammalian Groups 









Bio geographical Realm 






Group 


Neotropical 


Nearctic 


Palearctic 


Ethiopian 


Oriental 


Australian 


Monotremes 


_ 


_ 




_ 


— 




— 


2 


Marsupials 


+ 


1 




— 


— 




2 


+ 


Edentates 


+ 


1 




— 


— 




— 


— 


Bats 


+ 


+ 




+ 


+ 




4- 


+ 


Ungulates 


few 


+ 




+ 


+ 




+ 


— 


(Artiodactyls & 


















Perissodactyls) 


















Rodents 


© 


+ 




+ 


+ 




+ 


© 


Carnivores 


+ 


+ 




+ 


+ 




+ 


— 


Primates a 


+ 


— 




few 


+ 




+ 


— 


Insectivores 


few 


+ 




+ 


+ 




+ 


— 


Lagomorphs 


+ 


+ 




+ 


+ 




+ 


(intro- 
duced) 


Tapirs 


+ 


— 




— 


— 




+ 


— 


Camels 


+ 


— 




+ 


— 




— 


— 



+ -representatives of group are present. 

representatives of group are absent. 

1, 2 -only 1 or 2 species of group are present. 

©-group is well represented but species differ markedly from those in other parts of the world. 

a - exclusive of man. 

In Australia, very few groups are represented; the marsupials pre- 
dominate, and only the rodents and bats are well represented among the Eutheria. 
The only living egg-laying mammals or monotremes are found there. 

From this brief sketch of mammalian distribution, it is clear that the 
different regions of the world have their own distinctive faunas, though adja- 
cent regions tend to show more similarities than more remote areas. It should 
also be noted that the Chiroptera, the bats, are the only order rather uniformly 
distributed throughout the world. The widely distributed rodents have endemic 
groups (that is, peculiar to a particular locality) , especially in Australia and the 
Neotropical region. Similar findings have emerged from the study of other 
animal and plant groups. The above facts suggest that in addition to the ecolog- 



72 • THE EVIDENCE FOR EVOLUTION 

ical factors that set limits on distribution, the other major limiting factor on 
distribution is what we may term the historical. A species or group will only be 
present in a given region if, at some time in the past, it was able to reach that 
region. For most species, oceans or deserts or mountain ranges have been bar- 
riers to the further expansion of their ranges. The bats, however, with their great 
mobility, have spread easily throughout the world, even to the most remote 
oceanic islands. This explanation raises almost as many problems as it solves, for 
the implication is that each species has had a single center of origin. The ques- 
tions that arise in connection with any species are, then, where was its center of 
origin and when did it originate. 

Present distribution is intelligible only on the assumptions that each 
species has originated only once, that species have had their origins in practically 
all habitable parts of the earth, and that they have originated throughout the 
geological history of the earth. Each species tends to expand like a gas from its 
center of origin, the pressure being due to its high reproductive capacity; migra- 
tion will then fill all available areas until further expansion is blocked by 
physical barriers or by unfavorable environmental conditions. New species can 
evolve only after a population of an existing species has become to some degree 
physically isolated from the parental species. Hence related species or groups 
will tend to be found in adjacent areas. We will now consider some specific ex- 
amples of geographical distribution and see how they are explained in terms of 
the theory of evolution plus a knowledge of the geological history of the earth. 

Primitive and Modern Mammals in the Neotropical 

The mixture of "primitive" and "modern" mammals in the Neotropical 
region has already been mentioned. The "primitive" group includes anteaters, 
sloths, armadillos, many marsupials, primitive primates (platyrrhine monkeys 
and marmosets), and a unique group of rodents. All of these are peculiar to 
South America. The "modern" group is very similar to the fauna of North 
America though for the most part the species are different. Included are deer, 
various cats, wolves, otters, many rodents, guanacos, and llamas. 

With the assumption of evolution, the explanation is relatively simple. 
Marine fossils similar to those of the Miocene elsewhere are found on land in 
Panama; thus Panama, the link between North and South America, must have 
been submerged during the mid-Tertiary. The "primitive" group of mammals 
reached South America in the late Cretaceous and Paleocene from North America 
and then evolved in isolation during the period of submergence. Re-emergence 
of the land gave rise first to island chains and then Panama rose again above the 
surface of the sea during the Pleistocene. The "modern" mammals invaded 
South America via this new land bridge. Many of the "primitive" forms in 
South America could not compete with the more efficient new immigrants and 



GEOGRAPHICAL DISTRIBUTION • 73 

became extinct. Their history is known from the extensive fossil record. Only a 
few species of the South American fauna were adaptable enough to spread their 
ranges into North America, among them the armadillo, the opossum, and the 
porcupine. 

Nearctic and Palearctic 

The similarities in the biota of North America and Eurasia have already 
been mentioned as warranting the inclusion of both areas in one biogeographical 
realm, the Holarctic. Though these two land masses are now isolated, the evi- 
dence is clear that in the late Tertiary, a land bridge in the Bering Sea region 
was repeatedly formed and broken. The fossil record indicates that the camels 
originated in North America and flourished here, evolving into a variety of 
species, some of which migrated to South America or to Asia. At present the 
group is entirely extinct in North America, but the curiously disjunct distribution 
of the group is intelligible when the fossil record and geological events are 
known. However, migration more frequently was from Asia to North America; 
the bison, mammoths, bears, cats, and deer, for example, originated in the 
Eurasian land mass and spread to North America. It should be realized that 
during this period climatic conditions underwent changes as well. Early in the 
Cenozoic, North America was relatively flat, and the Bering land bridge formed 
a broad connection between the two continents. Fossils deposited at that time 
indicate that the climate was much milder than at present, for the fossil record 
shows that alligators, sassafras trees, and magnolias were more or less continu- 
ously distributed from southeastern United States to eastern China, from the 
banks of the Yangtse to the banks of the Suwanee. In the late Cenozoic, the 
Rockies rose, western North America became colder and drier, and these species 
were eliminated from much of their former range. Next came the glaciers, which 
wiped out practically everything in their paths. In North America their extreme 
southern limits were, roughly, the Ohio River and the Missouri River (see 
Fig. 7-2). During this invasion by the ice, southeastern United States and 
eastern China were only slightly affected, and in these two areas the alligators, 
the sassafras, and the magnolias survived. In the million years since these popu- 
lations became isolated from each other they have evolved to the extent that they 
are now recognized as distinct species of the same genus; there are other species 
(for example, skunk cabbage) that are apparently the same in both areas. 

Relict Alpine Populations 

In the Northern Hemisphere it is often observed that species at the 
higher altitudes in mountainous areas are similar to those at lower altitudes 
farther north rather than to the species living at the foot of the mountains. For 



74 • THE EVIDENCE FOR EVOLUTION 

North Pole! 




□ ' 



Ice pack 

% Not covered 
by ice 



Fig. 7-2. The approximate extent of the glaciers in North America during the 

Pleistocene. 

example, some of the species in the Great Smoky Mountains of Tennessee are 
only found again hundreds of miles to the north in Canada, and some plants on 
Mt. Washington in the White Mountains of New Hampshire are isolated popu- 
lations of species found in Labrador. It seems probable that species adapted to 
arctic or subarctic conditions retreated to the south as the glaciers advanced, and 
were forced into more southern areas of North America. As the glaciers re- 
treated, the species migrated north and also up the mountainsides, continuing to 
survive in areas to which they were adapted. In this way relict populations were 
left behind on the mountains. 



Primitive Southern Fauna 

Not only South America but the other southern land masses, Africa 
and Australia, have "primitive" fauna, each, however, quite unique. Australia's 
mammals are primarily marsupials, and among the insects are found primitive 
bees, termites, and butterflies. In Africa, "primitive" primates such as the lemurs, 
and other species such as the aardwolf and the chevrotain still exist. Each of 
the three areas has a genus of the lungfish. This concentration of primitive 



GEOGRAPHICAL DISTRIBUTION • 75 

species in the Southern Hemisphere has led some investigators to believe that 
these land masses were at one time united but later split apart and gradually 
drifted northward to their present positions. This interesting theory of "Conti- 
nental Drift" postulates that at one time there were two major land masses — 
Gondwana, centering on the South Pole, and Laurasia in the vicinity of the 
equator. These masses drifted gradually northward, Laurasia splitting into North 
America and Eurasia, and Gondwana splitting up to form Africa, South America, 
Antarctica, and the Arabian and Indian peninsulas. The drifting was very slow 
and not completed until the Tertiary. Although the fossils of tropical species in 
Alaska and the lungfish genera and other similarities between Australia, Africa, 
and South America could be explained on this basis, the geological evidence for 
the split is not impressive and the theory poses about as many biogeographical 
problems as it solves. 

Some form of Matthew's theory of climate and evolution seems a more 
reasonable explanation for the geographical distribution of living and fossil 
species. Matthew suggested that the continents and ocean basins have occupied 
relatively permanent positions at least since the Mesozoic, but that the climate of 
the earth has fluctuated between warm, moist periods and cold, dry periods. 
During the warm phases, the seas have covered the continental lowlands, and 
tropical and subtropical species have expanded their ranges far to the north. 
During the cold phases, the continents were elevated, glaciers expanded south- 
ward, and only the tropics remained mild. The land masses were primarily north 
of the equator, and the southern continents remained more or less isolated and 
warm even during the cold periods. In the glacial periods, species had to adapt 
to the changing conditions, or migrate, or perish. Major new evolutionary types 
seem to have appeared on the major land masses, the southern continents serv- 
ing as refuges. This theory explains geographical distribution, then, by means of 
climatic changes and known land bridges, with no major shifts in the position of 
the continents or the oceans. The most probable explanation of the fossil record 
appears to be that the earliest mammals — monotremes and marsupials — origi- 
nated in Eurasia or North America and were able to migrate into all the major 
land areas. When the placental mammals arose, also in the Northern Hemis- 
phere, they replaced the marsupials in the Holarctic; the former connection to 
Australia was completely broken, however, and Africa and South America were 
partially isolated by barriers of desert or water, and the more "primitive" forms 
there were at least partly protected from competition with the more "modern" 
and efficient mammals that continued to evolve to the north. 

Continental and Oceanic Islands 

Two distinct types of islands, continental and oceanic, can be identified. 

The continental islands are generally separated from a continent by a shallow 



76 • THE EVIDENCE FOR EVOLUTION 

sea. The rock formations on both the land mass and the island are similar, with 
the islands basically formed from stratified rock. The continental islands are 
separated from the mainland if the sea level rises or the land sinks. Typical of 
the continental type are the British Isles, Borneo, Sumatra, and Java. Oceanic 
islands are usually volcanic in origin, hence formed of igneous rock, and are sepa- 
rated from the major land masses by deep water. The Hawaiian Islands and the 
Galapagos Islands are examples of oceanic islands. Not only do continental and 
oceanic islands differ in their mode of origin, but they have quite different types 
of fauna. 

Each of the oceanic islands or island groups has its own distinctive 
fauna, different from the faunas in all other parts of the world. Compared with 
the continents, the oceanic islands have depauperate faunas. There are seldom 
any mammals except bats, though rodents, possibly introduced by man, are some- 
times present. The only fresh-water fishes are those capable of adapting to life 
in salt water. Such small animals as snails, lizards, insects, and land birds are 
found. The fauna of continental islands is clearly derived from the nearby conti- 
nent; though the species may sometimes be different, the similarities are quite 
striking. There is a distinct relationship between distance and the similarity of 
the species on island and mainland. The British Isles have essentially the same 
species as the European mainland; Ireland, however, lacks some elements found 
on the continent. Though St. Patrick has long received credit for the absence of 
snakes there, their inability to cross an ocean barrier in postglacial times is a 
more reasonable, though less romantic, explanation. Where the distance is 
greater or the connection to the continent less recent, as in Sumatra, Java, or 
Borneo, different species have had a chance to evolve, but they are similar to the 
mainland species that originally populated the island and from which they are 
descended. On Sumatra, for example, a small edition — a different species — of 
the rhinoceros found on the mainland has evolved. For some reason, island 
species are frequently smaller than their close relatives on the mainland, but the 
adaptive significance of this tendency requires further study. 

After a volcanic eruption the oceanic islands must have formed a 
barren mass of rock in the vast distances of the sea. The explosion of Krakatoa 
in 1883 has provided an actual example of such an event for study. Once 
formed, the island will become inhabited only by those species capable in one 
way or another of traversing the formidable barrier of ocean and sheer distance 
that confronts the terrestrial species. Chance thus plays a large role in determin- 
ing which species happen to bridge the gap. Some groups, however, are much 
more capable of wide dispersal than others; for example, the probability is great 
that such groups as birds and bats will be present, but it is practically zero for ele- 
phants. Among the birds, chance again may play a major role in determining 
which species reach the island. The Hawaiian honey creepers and Darwin's 
finches on the Galapagos Islands are instances of arrays of species that have 



GEOGRAPHICAL DISTRIBUTION • 77 

evolved on the islands from original immigrant groups, perhaps even a single 
flock wandering or blown far from its usual haunts. 

Thus, the present distribution of species is most intelligible if inter- 
preted in terms of the ecological conditions, the historical factors that have 
limited their expansion, and the theory of evolution. Within this framework, the 
peculiarities of island distribution, alpine distribution, regional similarities, and 
the many other facets of biogeographical distribution can be fitted. No other 
system has a logical, rational explanation for so many of the facts. 



► SUMMARY 



Plants and animals are not uniformly distributed over all 
parts of the world. The spread of many species is quite obviously 
limited by the prevailing ecological conditions. Nevertheless, the 
suitability or unsuitability of the environment is not alone a 
sufficient explanation for the distribution of the flora and fauna, 
for introductions have shown that many species can thrive far 
beyond the limits of their natural range. On the other hand, 
within a given land mass, even though a variety of habitats 
exists, the species tend to evidence many similarities despite their 
adaptation to different conditions. These facts are most easily ex- 
plained by the theory of evolution. Within a given region the 
variously adapted groups have evolved from a common ancestral 
stock; hence their underlying resemblances that made possible the 
identification of biogeographical realms. The changing, evolving 
species in one area can only spread into other parts of the world 
if there are no barriers to their expansion. Thus, distribution has 
an historical as well as an ecological basis. The details of conti- 
nental, alpine, and island distributions of living species have be- 
come increasingly well understood as knowledge of paleontology 
and past geological and climatic changes has increased. Neverthe- 
less, the theory of evolution is essential to a complete understand- 
ing of present-day distribution, for the species have obviously 
been dynamic and changing rather than static entities. 

SUGGESTED READING 

Cain, A. S., 1944. Foundations of plant geography. New York: Harper. 
Darlington, P. J., 1957. Zoogeography. New York: Wiley. 

Darwin, C, 1839. The voyage of the Beagle. New York: Bantam Books (1958). 
Du Toit, A. L., 1937. Our wandering continents. Edinburgh: Oliver and Boyd. 
Lack, D., 1947. Darwin's finches. New York: Cambridge University Press. 
Matthew, W. D., 1939. Climate and evolution, 2d ed. New York: New York 
Academy of Science. 



78 • THE EVIDENCE FOR EVOLUTION 

Simpson, G. G., 1950. "History of the fauna of Latin America," Amer. Sci., 38:361- 

389. 
, 1953. Evolution and geography. Eugene: Oregon State System of Higher 

Education. 
Wallace, A. R., 1876. The geographical distribution of animals, 2 vols. London: 

Macmillan. 

, 1911. Island life, 3d ed. London: Macmillan. 

Wegener, A., 1924. The origin of the continents and oceans, 3d ed. (J. G. A. Skerl, 
tr.) New York: Dutton. 






CHAPTER 



8 



Systematics 



Taxonomy is one of the oldest biological disciplines, but 
today it is increasingly being pushed into the background by the 
rapid developments in such fields as physiology, ecology, embry- 
ology, and genetics. Yet taxonomy remains as the foundation 
stone for all biological research simply because the starting point 
in any biological experiment is an organism, and in order to con- 
duct and describe an experiment properly, you must know and 
know with certainty what organism you are using. Otherwise, it 
may be impossible for you or anyone else to confirm or to dupli- 
cate your results. This fact has all too often been slighted or over- 
looked, particularly by experimental biologists, who may speak 
of using "liver" or "frog muscle" as if all livers and all frog 
muscles were alike. In at least one instance, a series of experi- 
ments was abandoned after it was found to be impossible to 
identify the organisms being used. 

Classification 

All of us are taxonomists to some extent, in that we 
learn to identify the animals and plants that we encounter fre- 
quently. Taxonomy, or systematics as it is often called, grew out 
of the study of local faunas and floras. As information accumu- 
lated, the taxonomic problems quickly became more complex than 
those encountered in a local, essentially nondimensional system. 
It is virtually a biological axiom that no two organisms are iden- 
tical. Yet it is also true that some organisms are much more alike 



79 



80 • THE EVIDENCE FOR EVOLUTION 

than others. The taxonomist's problem, essentially, is to seize upon the 
significant similarities and thus try to bring some sort of order out of 
this chaos of variation. Many different systems are possible. Plants, for 
example, may be grouped by the color of their flowers as is often done in 
popular flower guides, or by their habitats, or by their size, and so on. The 
method used, which is not quite so simple, is known as the "natural system of 
classification" and stems from Aristotle. It is based on the degree of similarity 
in morphological characters, for it has been found that many individuals are 
very much alike and can be grouped together as a species. All house cats, for 
example, belong to the species Felts catus. Certain species, in turn, are quite 
similar and hence are grouped together in a higher category, the genus. The 
house cat, Veils catus, the mountain lion, Felis concolor, and the lynx, Felis lynx, 
all belong to the genus Felis. Certain genera are much more alike than other 
genera and thus can be combined into a family; the genus ? anther a, which in- 
cludes the "big cats" such as lions, tigers, and leopards, together with the genus 
Felts belongs to the family Felidae. The family Canidae (dogs, foxes, and 
wolves), the Ursidae (bears), the Mustelidae (weasels, skunks, mink, etc.), the 
Felidae, and several other families are grouped together in a higher group, the 
order Carnivora, or the flesh eaters. The orders can be arranged in still higher 
categories, the classes and phyla, thus forming a complete hierarchy. Each family, 
for example, can be characterized by a constellation of traits that sets it apart 
from all other families and that describes not only each genus within the family, 
but each species, and even each individual. Hence, to assign a species to a par- 
ticular higher group characterizes it at once with respect to a certain combination 
of traits, and the problems of dealing with over a million different species are 
thereby greatly simplified. Even though this hierarchical pattern of variation was 
recognized and used for centuries, it remained a puzzle as to why organisms fell 
into this particular pattern rather than some other geometrical configuration. 

Variation 

At this point it is well to consider the nature of variation within groups 
of related individuals. First of all, it must be reemphasized that there is not a 
continuum in the pattern of variation. There are, for example, no individuals 
who are intermediate in their traits between a house cat and a dog. Even in cases 
where the resemblance is much closer than that between a dog arid a cat, inter- 
mediates do not exist. The thrushes of the genus Hylocichla are very difficult 
to identify in the field, but even though five different kinds — the veery, and the 
wood, hermit, olive-backed, and gray-cheeked thrushes — are found in the same 
region, intermediate types will not be found. Without now attempting a species 
definition, we say that there are five species, each composed of similar but not 
identical individuals. As in this case, species are for the most part quite distinct 
from each other. 



SYSTEM ATICS • 81 

Considerable variation may exist within a species, for within a given 
population two or more different expressions of a trait may appear, a type of 
variability called polymorphism. The most familiar example undoubtedly is a 
human population with its variety of sizes, shapes, eye and hair colors, and so 
on and on, but populations of other species show similar variability. Whether it 
be screech owls, deer mice, fruit flies, or turtles, variations may range from very 
minor differences to such a striking specimen as an albino snapping turtle. These 
differences between individuals may be either genetic or nongenetic in origin. 
Some differences are simply seasonal or age differences. The spring and fall 
plumage of many migratory birds and the differences between a caterpillar and 
a butterfly or a tadpole and a frog represent merely different stages in the life of 
the same individual; in some species like the aphids, seasonal generations exist. 
The impact of the environment can also cause wide variations. The form of 
corals in the surf is quite different from that found in quiet lagoons, and dande- 
lions growing in an alpine habitat differ in form from those in the valleys below. 
The hereditary variations include the differences between the sexes, which may 
be as striking as the presence and absence of wings in some insects or antlers in 
deer, as well as the great array of hereditary variations of greater or lesser degree 
to be found in all sexually reproducing populations. 

Though local populations are polymorphic, other patterns of variation 
emerge when wider areas are examined. A dine is said to exist when a trait or 
a group of characters is observed to change more or less continually and gradu- 
ally as one moves from one part of the species' range to another. The song 
sparrow, Melospiza melodia, is widely distributed and common in North Amer- 
ica but is by no means uniform throughout its range. In the prairies and in the 
arid regions of the Southwest the birds are paler in color; in the more humid 
regions to the east and up the Pacific coast the birds are duskier in color, the 
transition being more or less gradual even though at least 20 subspecies have 
been named. Where the species is broken up into more clearly defined geograph- 
ical races or subspecies, it is said to be polytypic (see Fig. 8-1). For example, in 
the Philippines a small kingfisher inhabits a number of the islands, but each 
island's population is isolated from and easily distinguished from that of the 
other islands. Man, too, is polytypic as well as polymorphic, for the human 
species is readily subdivided into three major geographic races, the Negroid, 
Mongolian, and Caucasian. 

The Binomial System 

Modern taxonomy stems from the 1758 edition of Sy sterna Naturae, a 
volume by Linnaeus, a Swedish botanist. The binomial system of nomenclature 
that he introduced was simple yet precise — two characteristics needed for a 
workable system. For example, a small fish can easily be singled out if it is 
known that it is pale brown "with a dark bar behind the opercles and 



82 • THE EVIDENCE FOR EVOLUTION 





Fig. 8-1. The bobwhite quail, a polytypic species. Each of the five males, shown 
in dorsal and ventral views, is representative of a different population in the United 
States or Mexico. All five, so distinctive in appearance, are considered to be mem- 
bers of the same species, Colinus virginianus. 



SYSTEM ATICS • 83 

across the dorsal and anal fins, which are bright orange in spring males. The 
lips are thick and fleshy. The intestine is very peculiar, it is wrapped many times 
around the swim bladder. The scales are 7, 49-55, 8. The dorsal fin has 8 rays, 
the anal fin 7. The teeth are 4-4. This species reaches a length of 8 inches." 
(Eddy and Surber) Though it is accurate, no one in his right mind would try 
to use this description in everyday conversation. And yet the common name, 
stoneroller, is no more satisfactory, for what is one man's stoneroller may be 
called stonelugger by another, or doughbelly, or even rotgut minnow. The more 
picturesque common names suffer from their lack of precision, but the binomial, 
Campostoma anomalum, is both precise and brief, and has been assigned to the 
"minnows" of the family Cyprinidae fitting the above description. 

At one time the scientific name was assigned to a single specimen, the 
type specimen, and all individuals collected subsequently were referred to it in 
order to determine whether they belonged to the same or a different species. One 
of the major advances in modern systematics is that the type concept has been 
almost entirely abandoned. The fallacy of the type concept can be easily made 
clear. Suppose, for example, you were told to go out and collect the type spec- 
imen for the species Homo sapiens. Would it be male, or female? If you could 
settle this question to your own satisfaction, how would you then decide which 
member of your sex to bring in? The basic facts of biological variation have 
made it abundantly clear that the type specimen is not typical of anything. The 
important point to determine is the range of variation in the species. For this 
purpose adequate sampling methods must be used so that statistical analyses can 
be applied. Hence, taxonomic studies are becoming studies of populations rather 
than of individuals. The type specimen has become the individual to which the 
species name is attached; in case what was originally thought to be one species 
later turns out to be two, the original name will be reserved for individuals 
similar to the type and a new name assigned to the other group. 

As mentioned earlier, the natural system of classification, stemming 
from Aristotle and formalized by Linneaus, with its hierarchy of taxonomic 
groups of different levels of morphological similarity was always something of a 
biological puzzle because it worked so well even though there was no obvious 
reason why this particular geometrical configuration should exist rather than 
some other. The publication of The Origin of Species in 1859 offered a simple 
solution to the puzzle — that is, the theory of evolution. When different species 
are similar, the similarities are due to descent from a common ancestry. The 
closer the similarities, the more recent the divergence and the closer the genetic 
relationship between the species. After Darwin, the natural system, based on 
morphological similarities, became a phylogenetic system based on degree of re- 
lationship. It might be expected that changing the criterion for classification 
would drastically change the classification system itself, but no major changes 
were necessary. Perhaps the main inference to be drawn is that the system of 



84 • THE EVIDENCE FOR EVOLUTION 

classification is not arbitrary but natural, reflecting the objective state of species 
in nature. And systematics has become more than classification; it has become the 
study of evolution. 

Some Taxonomic Problems 

Although the binomial system generally works beautifully, anomalous 
situations occasionally arise that are very difficult to resolve satisfactorily. For 
example, the purple grackle breeds in a belt between the Appalachians and the 
Atlantic from just north of New Jersey to Florida and southern Louisiana, and 
the bronzed grackle breeds in New England and in the St. Lawrence and Missis- 
sippi Valleys. Yet where the ranges of the purple and bronzed grackles meet, 
all along the Appalachians, they interbreed, and intermediate types of individuals 
are found. At present, the two groups are considered separate species, Quiscalus 
quiscula, the purple grackle, and Quiscalus versicolor, the bronzed grackle. 
Where such extensive interbreeding occurs over such a large area, it would 
seem just as reasonable to consider them as two subspecies of the same species, 
which replace each other geographically. 

A somewhat different situation exists in the leopard frog, Rana pipiens, 
the most widely distributed frog in North America, ranging from Mexico far 
into Canada. In this case it has been shown that when frogs collected in Florida 
or Texas are crossed with those from Wisconsin or Vermont, the hybrids are 
deformed and unviable. In other words, members of what is generally regarded 
as a single species are not even capable of interbreeding. 

One further instance may be cited. Butterflies of the genus ]unonia are 
distributed from Florida along the Gulf Coast, into Mexico and Central America, 
across northern South America, and up through the West Indies (see Fig. 8-2). 
The populations gradually change in their characteristics as one proceeds around 
the ring, but adjacent populations are similar and are capable of interbreeding. 
This ring of races, or Rassenkreis as it is often called, is closed in Cuba, for 
there butterflies resembling those in Florida coexist without interbreeding with 
butterflies like those to the south in the West Indies. In Cuba, then, these two 
populations behave like two distinct and well-defined species, yet there is no 
single place around the ring where it is possible to say that here one species 
stops and the other begins. 

For the taxonomist who is trying to work out a satisfactory scheme of 
classification, situations such as the three cited pose very real and very tricky 
problems — and there are many others even more complex. For the student of 
evolution, however, these taxonomic difficulties furnish still another argument in 
favor of evolution. If evolution is a gradual process that has been in progress 
through time, then indications that species are now undergoing change should 
be expected among living species. The existence of these puzzling taxonomic 



SYSTEM ATICS • 85 



In Cuba, Northern 
and Central races 
coexist without 
interbreeding 




Northern race 



Fig. 8-2. The distribution of geographic races of the butterfly Junonia lavinia 
(Precis lavinia) commonly known as the Buckeye. (Based on Forbes.) 



problems is evidence that species are not static, inflexible units, but rather are 
capable of change. The very hierarchy of genera, families, orders, and so forth is 
in itself evidence for the correctness of the theory of evolution, for that is the 
pattern that evolution should cause to develop. 



86 • THE EVIDENCE FOR EVOLUTION 



SUMMARY < 



At first acquaintance, the living world may seem a chaos 
of variation. It is, however, possible to bring order from this 
chaos, to arrange living things in a reasonable, workable system 
of classification. The "natural system of classification" that has 
developed, culminating in the Linnaean binomial system, is based 
on the degree of similarity in morphological traits. When ar- 
ranged under this scheme, living things fall into a hierarchy with 
the similarities becoming more specific at each level from phylum 
to genus. The theory of evolution furnished a cogent explanation 
for this pattern of variation. The similarities so readily observed 
are the result of descent from a common ancestry and are a reflec- 
tion of the actual genetic relationship between the species. The 
taxonomically difficult groups merely confirm the theory of evolu- 
tion, for the difficulties largely arise in groups that are in the 
process of diverging to become distinct species — clear evidence of 
the operation of evolution. 

SUGGESTED READING 

Eddy, S., and T. Surber, 1947. Northern fishes. Minneapolis: University of Minne- 
sota Press. 

Huxley, J., ed., 1940. The new systematic s. New York: Oxford University Press. 

Mayr, E., 1942. Systematic s and the origin of species. New York: Columbia Univer- 
sity Press. 

, E. G. Linsley, and R. L. Usinger, 1953. Methods and principles of system- 
atic zoology. New York: McGraw-Hill. 



CHAPTER 



Comparative Embryology 



Each individual starts his independent existence as a 
single cell, the fertilized egg or zygote. The hereditary material 
contained by each zygote is the surviving product of millions of 
years of evolution. Each zygote develops in an environment of 
some sort. The characteristics of the adult organism are deter- 
mined by the interaction between the developing embryo and its 
environment. Abnormalities either in the transmitted germ plasm 
or in the environment may cause abnormal development in the 
individual. The zygote itself is a spherical object bearing little or 
no resemblance to the adult form, which is only reached by 
gradual stages. The sequence of stages from the single cell to the 
adult and beyond — that is, the individual's developmental history 
from fertilization to old age — is known as the ontogeny of the 
individual. The various adult forms of an evolving species may 
also be considered as a series of stages in the history of the 
species, a series which is called its phytogeny. With two such 
series available, it was almost inevitable that someone would com- 
pare them. Haeckel, who made such a comparison, propounded 
the biogenetic "law" or the Theory of Recapitulation, which 
states, "Ontogeny recapitulates phylogeny." In other words, the 
embryo in its development retraces its evolutionary path, or 
climbs its family tree from the one-celled ancestor up to the 
present. The adult stages of ancestral forms are repeated, but they 
are now to be found in the earlier stages of ontogeny. For ex- 
ample, the stage early in development, in which gill slits are visi- 
ble in birds and mammals, was considered by Haeckel to be equiv- 



87 



88 • THE EVIDENCE FOR EVOLUTION 

alent to the adult fish ancestors in the phylogeny of these groups. Thus, evolution 
was thought to be occurring in the adult, with new adult forms being tacked on 
to the old at the end of the developmental period. This concept has had con- 
siderable appeal, especially to zoology professors, for the zygote could be com- 
pared to the single-celled protozoan ancestor, the blastula to a colonial flagellate 
such as Volvox, the gastrula stage to a two-layered coelenterate like Hydra, and 
so on. Phylogeny then became not only the explanation but the cause of 
ontogeny, a conclusion that actually hampered research into the causative mechan- 
isms in development. 

von Baer's Dicta 

Haeckel's generalization was too sweeping. The earlier statements of 
von Baer, though less striking, were more accurate. He had observed that in de- 
velopment the general traits appear before the more specialized, that the embryos 
of different species are more alike than the adults and depart progressively from 
each other during ontogeny, and that the young stages of a species are not like 
the adults of species lower in the phylogenetic series but rather like their embry- 
onic stages. There is a germ of truth in the biogenetic law even though it is 
demonstrably false if taken too literally; hence it would be more proper to say, 
though von Baer did not, that "Ontogeny recapitulates ontogeny." Vertebrate 
embryos do show many similarities, for which the most reasonable explanation is 
their common ancestry. 

In the development of the mammalian heart, for example, the number 
of chambers is initially two, then three, and finally in the adult, four. The mam- 
malian phylogeny includes first the fishes with a two-chambered heart, then the 
amphibians with three, and the reptiles with four. The basic number of aortic 
arches in vertebrates is six, the living fishes having arches 3 through 6 complete 
and traces of the first two. These arches break up into capillary beds in the gills 
and then regroup to form the dorsal aorta. The lower amphibians have arches 
3 through 6, but the lower part of the 6th aortic arch has now become the 
pulmonary artery to the lungs. In the higher amphibians and reptiles the 5th 
arch is also missing in the adult, the 3rd becomes the carotid arteries to the 
head, the 4th, the systemic arteries to the rest of the body, and the 6th remains 
pulmonary in function. In the adult mammals only the 3rd, the left half of the 
4th arch (in birds, the right half), and the lower part of the 6th are all that 
remain functional of the six arches that make their transient appearance during 
development. (See Fig. 9-1). 

Man's evolutionary past sometimes manifests itself in strange ways. 
From time to time we read of so-called "blue babies," who are suffering from 
insufficient oxygenation of their blood. There are two major causes for this con- 
dition: either the opening between the right and left auricles of the heart does 



COMPARATIVE EMBRYOLOGY • 89 

Carotid 




Ventral aorta 
■Dorsal aorta 

HYPOTHETICAL SHARK 

ANCESTRAL TYPE 






Pulmonary 
artery 



FISH 
(Teleost) 



AMPHIBIAN 
(Urodele) 






AMPHIBIAN 
(Anuran) 



REPTILE 



BIRD 



OJl 
MAL 



Fig. 9-1. Diagram of the evolution of the aortic arches in the vertebrates (ventral 

views). 



not close, or the duct of Botallus, a vessel connecting the pulmonary artery 
directly to the dorsal aorta, fails to close. Both opening and duct are devices by 
which the blood of the fetus is shunted past the nonfunctional lungs prior to 
birth. Since the opening between the auricles represents a persistence of the 
ancestral two-chambered fish heart and the duct of Botallus is actually the upper 
half of the 6th aortic arch, these blue babies are living evidence of man's evolu- 
tionary past. 



90 • THE EVIDENCE FOR EVOLUTION 

The gill arches and the gill slits in the mammalian embryos do not 
represent the adult ancestral fish, but are similar to those of a fish embryo at a 
comparable stage of development. They then differentiate into structures quite 
different from those in the fish. All of the gill slits close and disappear except 
the one that forms the Eustachian tube, which connects the pharynx at the back 
of the mouth to the middle ear. The gill arches themselves have a variety of 
fates. In the most primitive jawless fishes, of which the lamprey is a surviving 
relict, the gill arches number seven. The first arch became the basis for the jaws 
in the fishes, but the bones forming the jaw articulation in fishes, the quadrate 
and the articular, by an unusual turn of events have moved into the middle ear 
of the mammals during the course of evolution. There, as the incus (or anvil, 
formerly the quadrate) and the malleus (or hammer, formerly the articular), 
they form two thirds of the chain of small bones that conduct sound across the 
middle ear to the inner ear. The third bone in this chain, the stapes or stirrup, is 
derived from the second gill arch, which as the hyomandibular in fish more or 
less anchors the jaws to the brain case. The rest of the 2nd gill arch forms the 
body and the anterior horn of the hyoid apparatus, the posterior horn coming 
from the 3rd gill arch. The hyoid apparatus and other cartilaginous structures in 
the throat region such as the thyroid, arytenoid, and cricoid cartilages, derived 
from the 4th and 5th arches, are relatively insignificant compared to their size 
and functional importance in fish. (See Fig. 9-2.) All of the above statements 
are well grounded on embryological and anatomical evidence. The obvious ques- 
tion is why there should be a stage in the mammalian embryo where gills and 
gill arches, which never function as such, are nevertheless present, even though 
they differentiate into quite different adult structures. The most obvious answer 
is that the mammals are descended from fishlike ancestors and that in the course 
of evolution modifications in development have occurred; the similarities which 
still persist in the ontogeny of fish and mammals are indicative of a funda- 
mental similarity in their genotypes due to their common ancestry. 

Modifications of Development 

The notochord, characteristic of the Phylum Chordata, to which the 
vertebrates belong, is crowded out by the vertebrae almost as soon as it is formed 
in the vertebrate embryo. Why, then, is the notochord retained? It might seem 
to be a clear-cut case of recapitulation, but this can hardly be so. The cells that 
form the notochord are intimately bound up with the organizing and inducing 
of the essential axial structures of the embryo — the spinal cord and brain, the 
heart, kidneys, muscle, and so on; thus if this function is to be retained, the 
cells themselves must be retained. Because natural selection acts on living organ- 
isms at all stages of their existence, not just upon the adults, embryonic as well 
as adult stages and structures may be changed, added, or eliminated. Since selec- 
tion must act within the limits imposed by the modifications possible in already 
existing stages, the retention of stages similar to those of ancestral forms is to be 



COMPARATIVE EMBRYOLOGY • 91 



Gill slit Brain case 




Hyomandibular 
Palaroquadrate 



*"*••—.?.... 4 5 6 7 
PRIMITIVE JAWLESS TYPE 




Meckel's 
cartilage 



SHARK 



^•- Columella (stapes) 
Quadrate 




AMPHIBIAN 
OR REPTILE 



Gill arch 1 

(upper jaw of shark, 

palatoquadrate) 

Gill arch 1 

(lower jaw of shark, 

Meckel's cartilage) 

Gill arch 2 
(hyomandibular of 
shark; hyoid) 



Gill arches 3-7 



Thyroid cartilage 
Cricoid cartilage 



Tracheal cartilages- 

Fig. 9-2. Evolution of the gill arches in vertebrates. 



Styloid process 
of hyoid 
Meckel's cartilage 

— Ligaments 

Hyoid 

Larynx 
OJI 

HUMAN 



expected even though their subsequent developmental fates may differ. Many 
kinds of modifications of developmental patterns may be observed. 

In the typical frog, for example, the small eggs laid in water hatch after 
a few days into free-living, gill-breathing tadpoles that metamorphose after 
several weeks or months — or even years, in the bullfrog — into the adult frog. 
In the Hylodes of the West Indies, however, the large eggs laid on leaves hatch 
in two or three weeks directly into frogs, although a brief tadpole stage exists 



92 • THE EVIDENCE FOR EVOLUTION 

prior to the hatching of the frogs from the eggs. The elimination of the func- 
tional tadpole stage has taken place, but the tadpole nevertheless continues to 
appear; thus, although a secondary modification of the basic plan of frog devel- 
opment has occurred, the change has not been sufficiently drastic to eliminate the 
stage completely. Such information is evidence not only for evolution, but for 
its gradual nature. 

The fossil evidence and other evidence make it abundantly clear that the 
Amphibia are ancestral to the reptiles, birds, and mammals. The three latter 
groups are known as the amniotes, for their embryos develop within the watery 
cradle made possible by embryonic membranes known as the amnion and 
chorion. Yet since the amphibians lack these membranes, they must be new 
structures evolved during the evolution from amphibians to reptiles. In the 
mammals, a modification in function led to the utilization of the chorion as a 
part of the placenta. Thus new structures or modification of existing structures 
for new functions can evolve in the embryo as well as in the adult. 

In some instances precocious sexual maturity has led to the elimination 
of the adult stage, a phenomenon known as paedogenesis. In the axolotls, 
salamanders of the genus Ambystoma having a gill-breathing, water-dwelling 
larval stage, the larvae may mature sexually and reproduce without undergoing 
metamorphosis. That this is an example of paedogenesis is proved by the fact 
that the axolotl, under certain environmental conditions, metamorphoses into the 
adult lung-breathing, land-dwelling form. Compared to the other primates, man 
has an extended developmental period; in fact, human adults show more resem- 
blance to immature anthropoids than to the adult great apes. The lack of hair 
and of well-developed brow ridges, the relatively flat face, and the slow closure 
of the skull sutures have all been singled out as indicative of a tendency toward 
paedogenesis in man. 

New and different stages in the life cycle have also evolved. Among the 
primitive insects, the immature forms are rather similar in appearance and func- 
tion to the adults or imagoes. In the more recent groups of insects, the egg 
hatches into a larva quite different in form, function, and, usually, habitat from 
the adult into which it later metamorphoses. The caterpillars that become butter- 
flies and the squirming maggots that, after a quiescent pupal stage, emerge as 
flies, are familiar examples of insect metamorphosis. An example can even be 
cited much like Haeckel's concept of evolution: in the development of the crab, 
the megalopa stage resembles a lobster or crayfish, near relatives of the crabs, 
and the adult crab, with abdomen folded under, is a stage that appears to be 
tacked on to the ancestral form. 

Thus, it is clear that many changes in ontogeny have occurred: new 
embryonic stages not affecting the adults, for example, parasitic larvae of free- 
living adults; wide divergence of adults with similar embryos, for example, fish 
and mammalian embryos; adult forms that may resemble larval stages of ances- 
tors, that is, paedogenesis; or appearance of a new adult stage apparently added 



COMPARATIVE EMBRYOLOGY • 93 

to the previous adult stage. These changes must be due to the action of natural 
selection, producing changes in relative rates of development of various struc- 
tures as well as modifications in the function and structure of existing stages and 
structures. Where repetition of ancestral stages occurs, it is not simply a case of 
Haeckelian recapitulation, but rather an indication that similar groups of genes 
are operative and that the embryonic structures they control are still essential to 
normal ontogeny, and hence have not been eliminated by natural selection. 
Therefore, the study of embryology is helpful in determining relationships, and 
the rejection of Haeckel's dictum does not imply a rejection of all embryological 
evidence relating to evolution, for similarities in ontogeny are often indicative 
of phylogenetic relationship. In fact, they may often be the best evidence avail- 
able. In the free-living shrimp (Penaeus), the sessile barnacle (Lepas), and 
Sacculina, a parasitic sac in the crab, the Nauplius larval form of all three is the 
best evidence that these three diverse adult types are members of the Crustacea. 
Here and in many other instances, similarity in ontogeny is an indication of 
genetic affinity but is not necessarily evidence as to the adult form of the 
ancestors. 



► SUMMARY 



Despite the diversity of form among such groups as fish, 
amphibians, reptiles, birds, and mammals, the embryos of all of 
these vertebrates look very similar and have many features such as 
gill slits, aortic arches, neural tube, and notochord in common. 
Thus, the adult diversity results from the modification during de- 
velopment of the same basic embryonic plan. The assumption that 
these groups are all descended with modification from a common 
fish ancestry renders this situation intelligible. Other theories are 
quite inadequate to account, for example, for the presence of gill 
slits in birds and mammals, which never at any stage in their life 
cycle require functional gills. The recapitulation theory of 
Haeckel, as originally stated, represents an oversimplification of 
the facts, for the developing embryo does not recapitulate the adult 
stages of its ancestors. Rather, the embryo will in most instances 
show more resemblance to the embryos of ancestral or related 
groups than it will to their adult forms. For this reason compara- 
tive embryology can be a fruitful source of phylogenetic informa- 
tion. The evidence indicates that evolution must operate within 
the framework and limitations imposed by existing patterns of 
development. Although the end products in some cases have been 
as diverse as a fish darting through the water and a bird soaring 
in the sky, their embryos still carry the clues to their common 
ancestry. 



94 • THE EVIDENCE FOR EVOLUTION 

SUGGESTED READING 

DeBeer, G. R., 1958. Embryos and ancestors, 3d ed. New York: Oxford University 
Press. 

Nelsen, O. E., 1953. Comparative embryology of the vertebrates. New York: 
Blakiston. 

Willier, B. H., P. A. Weiss, and V. Hamburger, eds., 1955. Analysis of develop- 
ment. Philadelphia: Saunders. 



CHAPTER 



10 



Comparative Anatomy 



The similarity between different species was one of the 
fundamental reasons for the development of the theory of evolu- 
tion, and comparative anatomy has been one of the cornerstones 
of evidence for the theory ever since Darwin's time. In a sense, 
comparative embryology and comparative anatomy are one and 
the same study, differing only with respect to the stage of devel- 
opment of the organism, but historically and traditionally two 
disciplines have existed rather than one. Unfortunately, not all 
similarities between members of different species are due to a 
common ancestry, and the concept has sometimes been consider- 
ably overworked. Lamarck and especially St. Hilaire argued that 
all animal species conformed to a common archetype, a clearly 
erroneous idea that was strongly and effectively attacked by 
Cuvier. The fallacy of the archetype concept can be seen through 
a comparison of such "higher" animals as a mammal, an insect, 
and a mollusk like the snail; neither in general nor in particulars 
can they be truly said to conform to a common pattern at any 
stage. Lamarck's adherence to this concept undoubtedly weakened 
his arguments for evolution and may well be responsible for the 
fact that we now associate the theory of evolution with Darwin 
rather than Lamarck. 

Homology and Analogy 

There are apparently two major reasons for similarities 
between species — heritage and habitus. Heritage refers to a com- 



95 



96 • THE EVIDENCE FOR EVOLUTION 

mon ancestry, with similar genetic systems responsible for the resemblances. 
However, species with similar modes of life are often very much alike even 
though not closely related. The mechanism responsible for this type of simi- 
larity is natural selection, similar selection pressures bringing about similar 
adaptations to similar environments. The problem, of course, is to be sure that 
relationships attributed to heritage are not actually due to habitus, a distinction 
not always easily made. Two concepts have arisen in connection with these 




r : -\ 





TkWtott 



Fig. 10-1. Analogy. (From Animal Analogues by R. W. Wood.) 



differences that aid in clarifying the ideas involved; structures that are similar 
because of similar function or habitus are said to be analogous, whereas struc- 
tures that are similar because of common ancestry and a similar genetic basis are 
said to be homologous. 

The wings of a swallow and a dragonfly, though used by both in flight, 
are analogous since their origin and structure are clearly different. The fins of a 
trout and a dytiscid water beetle are also analogous. In both of these examples 
the structural differences between the vertebrate and the insect are fairly obvious, 
but this is not always the case. The camera-type eye with a focusing lens and a 
sensitive pigment layer has appeared in two groups of animals, the vertebrates 
and the cephalopod mollusks such as the squid and the octopus. The physical 
requirements for this type of eye are such that they must be quite similar struc- 



COMPARATIVE ANATOMY • 97 

turally if the eye is to function at all. Both have a lens, a sensitive pigment 
layer, and a layer of nerves, all housed in a spherical chamber, and superficially 
are much alike. However, the embryology of the eye in the two groups is quite 
different. Most striking, perhaps, is the fact that the vertebrate eye is, in a sense, 
arranged backward; that is, the layer of nerves carrying the impulses to the brain 
lies in front of the pigment layer rather than behind it, the latter being a more 
sensible arrangement and the one that is found in the cephalopod eye. It is clear 
from these examples that a similar problem, whether it be flying, swimming, or 
seeing, is apt to have similar solutions in different groups. Even though, at the 
outset, the heredity may be very different, the end products of the operation of 
natural selection are much alike. The evolution of widely divergent groups to- 
ward greater similarity due to common functions or adaptations is known as 
convergent evolution. The resemblances, however, are always superficial. 

Homologous structures, on the other hand, may or may not function 
alike; homology rests not on function but on a similar developmental origin and 
hereditary basis. A human hand, a bat's wing, and a cat's forepaw, for example, 
are homologous, for all are five-toed (pentadactyl) structures, functionally quite 
different, but of similar location and embryology in three different mammals. 

The distinction between homology and analogy may seem relatively 
clear-cut, but cases do arise where the decision will depend on point of view 
rather than any fixed criterion. The wing of a bird, the wing of a bat (a mam- 
mal), and the wing of a pterosaur (a flying reptile) are all derived from the 
vertebrate tetrapod forelimb and are thus homologous, in one sense. However, 
flight originated independently in these three groups, and the three types of 
wings are quite different in the details of their structure. In the bat wing all five 
digits of the pentadactyl forelimb are present. The wing of a bird utilizes only 
digits 1, 2, and 3, and in quite a different manner, with the fourth and fifth 
digits completely lost. The pterodactyl had four digits, with only the fourth 
elongated to support the wing and the fifth missing (see Fig. 10-2). With re- 
spect to their adaptations for flight, then, these wings should more properly be 
regarded as analogous rather than homologous. 

Homologies in Vertebrates 

Obviously, it is not possible to explore in detail the great wealth of 
material on comparative anatomy that has been amassed for many different 
groups. Volumes have been written even for a single group such as the verte- 
brates (see references at end of chapter) . Careful study of these texts and first- 
hand experience with the organisms themselves give an extremely convincing 
demonstration of the reality of evolution. However, some selected examples will 
serve to illustrate the nature of this type of evidence. 

Characteristically there are seven cervical vertebrae in the mammalian 



98 • THE EVIDENCE FOR EVOLUTION 

neck; a mouse, an elephant, and even a giraffe have the same number of cervical 
vertebrae. These mammals have a defined neck region and are capable of turning 
their heads, whereas the porpoise, a mammal with the torpedolike shape charac- 
teristic of the fishes, lacks a distinguishable neck region and cannot turn its head. 
Nevertheless, the seven cervical vertebrae are present in the porpoise although 
they are much shorter than in mammals of comparable size and are fused to- 




Fig. 10-2. Homology in vertebrate wings. 



gether so that flexibility has been lost. To the obvious question as to why animals 
differing so greatly in size, in structure, and in mode of life should have the 
same number of vertebrae in their necks, the theory of evolution presents a 
simple, plausible answer. All these varied forms, and the many other mammals, 
are descended, with modifications, from an ancestral mammalian stock that was 
characterized by seven cervical vertebrae. 

The evolution of the vertebrate skull, in which homologies have been 
traced from the fish up through the amphibians and the reptiles to the present- 



COMPARATIVE ANATOMY • 99 

day mammals, illustrates the amount of change that has taken place in the many 
millions of years of vertebrate history. The mammalian skull, an apparently uni- 
tary structure, has been shown to have been formed from three quite distinct 
components found in the fish skeleton: the endoskeletal brain case, the dermal 
bony armor in the head region, and the visceral skeleton supporting the gill 
arches (see Fig. 10-3). The original braincase housed the major sense organs — 




MAN 



Nasal = N 

Maxillary =M 

Dentary = D 

Frontal = F 

Parietal = P 

Jugal = J 

Temporal = 7 

Occipital =0 




LION 



Fig. 10-3. Homology in the bones of the skull. 



of sight, hearing, and olfaction — and was shielded by a complete roof of dermal 
bones imbedded in the skin. The jaws were originally derived from the gill 
arches. By a series of extensive changes involving modification, fusion, or loss 
of the bones in the fish skull, the mammalian skull such as that of the cat has 
arisen. Although the homologies between the fish and cat skull are by no means 
obvious without adequate study of the many forms representative of the numer- 
ous intermediate stages, and many people find it difficult in any event to accept 
that modern man's gum-chewing jaws are derived from structures that originally 



100 • THE EVIDENCE FOR EVOLUTION 

supported the gills of fish, the homologies between the cat (Felts catus) and the 
lion (Panthera leo) skulls are quite clear. The homologies are not so obvious 
between these skulls and that of man, in a different mammalian order, but study 
of the diagrams will show the many similarities between them. 

The pentadactyl appendage has already been mentioned as the character- 
istic condition in tetrapods, but not all tetrapods have five toes on each ap- 
pendage, and it may be questioned whether some of them ever did have five 
toes. In addition to the embryological evidence and the vestiges of digits that 
indicate the previous presence of additional digits, another type of evidence, 
from guinea pigs, is now available. The guinea pig has four toes on each fore- 
foot, but only three on each hind foot; a hereditary variant, called pollex, has 
been discovered that produces the five-toed condition on all four feet. Though it 
could be argued that such a mutation has no evolutionary significance, it seems 
more reasonable to suppose that it has restored the ancestral condition, and in 
any case it certainly establishes that guinea pigs can have five toes. 

Genetic Homology 

Morphological homologies are actually based on homologies in the 
hereditary materials or genotypes of different species, of which they are the most 
obvious manifestations. It is therefore significant that when it has been possible 
to study genetic homologies more directly, homologous genes have been demon- 
strated in closely related species. In different species of flies of the genus 
Drosophila, similar mutations affecting eye color, body color, the bristles, and 
other traits have been shown to exist. The homologies have been based not only 
on the similarities in phenotype, but on the location of these genes in homo- 
logous regions of the chromosomes and in some cases by crosses as well. 

Serial homology is a somewhat different concept from the one we have 
been considering, but it, too, has evolutionary significance. The segmented ani- 
mals such as the vertebrates and the arthropods are composed of a series of 
segments, each of which is basically similar to the others, and the structures in 
one segment can be compared and homologized to those in other segments. Serial 
homologies are clear-cut in an animal like the earthworm, an annelid, where 
most of the segments are replicas of each other. Even in arthropods such as the 
lobster and crayfish in which considerable differentiation of the segments has 
occurred, the homologies between various appendages such as the mandibles, the 
legs, the claws, and the antennae are easy to visualize. The segmentation of many 
insect larvae shows relatively little differentiation, and the homologies are there- 
fore easily established; but in adult insects, the great degree of differentiation 
serves to mask not only the homologies but even the segmentation itself to some 
extent. Nevertheless, in the insects the mouth parts, the antennae, and the legs 
have been considered to be serially homologous despite their dissimilarity in 



COMPARATIVE ANATOMY • 101 

appearance and function. The discovery of the so-called homeotic mutants in 
Drosophila has tended to reinforce these conclusions. The aristapedia mutant 
causes the development of a leglike structure in place of the antenna, and 
proboscipedia causes a similar change in the proboscis. Thus, the homeotic mu- 
tants cause one of a series of parts to assume the character of another member of 
the series, and by demonstrating the common potentialities of these varied ap- 
pendages have tended to confirm the conclusions previously drawn. 

In mo|t orders of insects there are two pairs of wings located on the 
second and third thoracic segments. In the two-winged flies of the order Diptera, 
the second segment bears the single pair of wings and the third bears the 
halteres, a pair of gyroscopic devices. The inference that the halteres are homo- 
logous (and serially homologous) to wings has been strengthened by the dis- 
covery of the homeotic mutants tetraptera, which produces a four-winged 
dipteran, and tetraltera, which causes flies with four halteres to develop. The 
discovery of mutants that change the ordinal characters of individuals carrying 
them has led some students, notably Goldschmidt, to believe that the higher 
taxonomic groups have originated in this fashion, an interesting speculation that 
does not appear, however, to be borne out by the facts. 

Vestigial Organs 

Another type of evidence for evolution is derived from the so-called 
vestigial structures. Not only do they suggest relationships, but they also raise 
questions about the mechanism of evolution; many vestigial organs have lost 
their adaptive function, and it may well be asked why they should continue to 
persist. Man himself is virtually a walking museum from his head to his feet. 
Many people, for example, have small nodes on their ears, known as Darwin's 
points, which are thought to be vestiges of the somewhat larger and more 
pointed ears of our ancestors. And even though we can no longer rotate our ears 
to test the sounds carried by each vagrant breeze as do the deer, nevertheless 
vestiges of these muscles remain that permit small boys and gentlemen at parties 
to show off by wiggling their ears. Human facial contortions are controlled by 
the remnants of the muscles with which our remote fish ancestors aerated their 
gills. When cold, our mammalian relatives fluff out their fur to increase the 
insulation of their bodies; we get goose pimples or duck bumps under the same 
conditions, but the attempt is abortive, for even though the muscles for fluffing 
the hair are present, the hair itself has virtually no insulating capacity. When 
angry or excited or frightened, your dog may raise the hackles along his neck, 
something we also try to do when we get the "chills" in a horror movie. The 
appendix and the coccyx are classical examples of human vestigial organs. The 
coccyx is all that remains of our tail, and the appendix seems to be of more 
trouble than value as an adjunct to the human intestine. Even the human foot- 



102 • THE EVIDENCE FOR EVOLUTION 

print, showing the arch and the big first toe, is a vestige of our simian ancestry 
and our former habitat in the trees. 

The theory of evolution gives a simple explanation for the presence of 
vestigial structures. The presence of a pelvic girdle in the python and the whale, 
a reptile and a mammal respectively, neither of which has hind limbs, is clear 
evidence that they are descended from tetrapod ancestors. Any other explanation 
is extremely difficult to apply or to accept. 



SUMMARY <■ 



Comparative anatomy rests on the distinction between 
homology and analogy. Homologous structures have a similar 
developmental origin and hereditary basis, but may or may not 
have a similar function. Analogous structures, though functionally 
similar, are otherwise different. The existence of many organs 
diverse in function yet clearly similar in structure — for example, 
the human hand, a seal's flipper, and a bat's wing — constitutes a 
conundrum best explained by evolution. The list of morphological 
homologies can be almost endlessly extended, but the interpreta- 
tion remains the same — namely, descent with modification. The 
persistence of nonfunctional vestigial organs of all kinds is still 
another biological phenomenon best accounted for by the theory 
of evolution. The serial homologies demonstrated in segmented 
animals are indicative of the evolution of segmental diversifica- 
tion from more uniformly segmented ancestral stocks. The as- 
sumption that anatomical homology and genetic relationship go 
hand in hand has been strongly reinforced by the discovery of 
homologies at the level of the chromosomes and the genes. 

SUGGESTED READING 

Davis, D. D., 1949. "Comparative anatomy and the evolution of the vertebrates," 

Genetics, paleontology and evolution. G. L. Jepsen, E. Mayr, and G. G. 

Simpson, eds. Princeton: Princeton University Press. 
Gregory, W. K., 1951. Evolution emerging, 2 vols. New York: Macmillan. 
Romer, A. S., 1955. The vertebrate body, 2d ed. Philadelphia: Saunders. 

, 1959. The vertebrate story, 4th ed. Chicago: University of Chicago Press. 

Spencer, W. P., 1949. "Gene homologies and the mutants of Drosophila hydei," 

Genetics, paleontology and evolution. G. L. Jepsen, E. Mayr, and G. G. 

Simpson, eds. Princeton: Princeton University Press. 
Young, J. Z., 1950. The life of vertebrates. Oxford: Clarendon Press. 



CHAPTER 



u 



Comparative Biochemistry 



Some biochemical traits are so fundamental that they are 
universally present in living things; others are widespread, char- 
acterizing large groups of animals or plants; still other bio- 
chemical properties are species specific or may even be unique to 
a given individual. Within this array of similarities and differ- 
ences is to be found considerable evidence for evolution and for 
the solution of specific phylogenetic problems. The term "homol- 
ogy" is customarily associated with morphological characteristics, 
but biochemical as well as structural homologies can be recog- 
nized. Common ancestry may be indicated just as clearly by 
homologous biochemical compounds as by homologous morpho- 
logical structures. This type of evidence, which gives essentially 
an independent check on the conclusions drawn from comparative 
studies in embryology and anatomy, was unavailable to Darwin. 
Since biochemical traits generally seem to change more gradually 
than morphological traits, the conclusions drawn from biochem- 
ical evidence are apt to be more soundly based. In some cases, 
biochemical evidence has made it possible to trace relationships 
where previously no reliable conclusions could be drawn from 
morphology. As might be expected, analogous biochemical com- 
pounds also exist; for example, both hemoglobin and hemocyanin 
function as oxygen-carrying respiratory pigments, but they are 
analagous rather than homologous, for hemoglobin is an iron- 
porphyrin protein whereas hemocyanin is a copper protein. 

Although different species may differ radically in their 
gross morphology, nearly all of them are formed from similar 



103 



104 • THE EVIDENCE FOR EVOLUTION 

compounds, which are used metabolically in similar ways. An elm tree 
and an elephant, a bacterium and a Bantu may at first glance appear 
to have little in common, but at the biochemical level they are much alike. 
The hereditary materials in both plants and animals, for example, are nucleic 
acids, while the stucture of the organism is erected primarily with protein mole- 
cules. The carbohydrates and fats, on the other hand, serve as the major sources 
of energy for carrying on metabolic work. The photosynthetic process makes 
possible the nutritional independence of the green plants, which are able to 
synthesize organic compounds (carbohydrates, fats, proteins, nucleic acids, etc.) 
from simple substances such as carbon dioxide, water, and inorganic salts. Other 
organisms, with few exceptions, are either directly or indirectly dependent on 
green plants for their energy. Even for a top carnivore (which does not serve as 
prey to another carnivore) such as a polar bear, this relationship can be traced 
back through the food chain to its origin in the chlorophyll of green plants. 
Despite the diversity of form and function found among the different species of 
plants and animals, certain chemical compounds play similar key roles in their 
metabolism. In the digestion of carbohydrates in animals, the complex polysac- 
charides are hydrolyzed and broken down into their constituent simple sugars or 
monosaccharides, of which the most important is glucose. The glucose molecules, 
after absorption from the intestine, become the building blocks for the formation 
of the animal's carbohydrates such as glycogen or, by stepwise oxidation, they 
become the major source of energy for the variety of processes going on within 
the cells. Similarly, proteins are broken down to amino acids, and fats to fatty 
acids and glycerol, which then, after absorption, enter into the metabolism of the 
animal. Furthermore, these substances are to a large extent interconvertible. The 
amino acids, for example, may undergo deamination or loss of the amino group, 
which then contributes to urea formation. The deaminized portion may be oxi- 
dized, ultimately to carbon dioxide and water, or it may be synthesized into 
glucose or a fatty acid or even into another amino acid. Thus, although the types 
of carbohydrates, fats, and proteins in different species are distinctive, many of 
the amino acids, fatty acids, and simple sugars of which they are composed are 
identical in both plants and animals. The metabolic pathways they follow are 
also similar. For example, the ornithine cycle, the Krebs tricarboxylic acid cycle, 
the cytochrome system, the metabolism of aromatic amino acids, glycolysis, the 
roles of actomyosin and adenosine triphosphate (ATP), and many other meta- 
bolic sequences have been identified in a wide variety of species. For this reason, 
it is possible to study cellular or general physiology, a field that concentrates on 
the phenomena common to the cells of many different species. The conclusion 
seems inescapable that the existence of these fundamental similarities must be 
regarded as evidence for an underlying kinship among all living things. It seems 
advisable, therefore, to examine in further detail the biochemical evidence relat- 
ing to evolution. 



COMPARATIVE BIOCHEMISTRY • 105 



Plant Pigments 



Some rather interesting information about evolution can be derived 
from a consideration of various plant pigments. Chlorophyll is present in all 
photosynthetic organisms, and this biochemical common denominator seems indic- 
ative of an affinity among these species. Several types of chlorophyll have been 
identified, but all have the same basic porphyrin or tetrapyrrole structure with 
magnesium attached to the ends of the pyrroles : 



2 n 5 




Chlorophyll a occurs in almost all types of photosynthetic organisms, but the 
other kinds of chlorophyll have a more limited distribution (see the listing be- 
low) . Even the sulfur bacteria contain chlorophyll-like compounds. 



group of plants 


chlorophylls 


green plants 


a and b 


brown algae 


<zand c 


diatoms 


a and c 


red algae 


a and d 


yellow-green algae 


a and e 


blue-green algae 


a 



The chlorophylls are bound to proteins in the chloroplasts and differ from each 
other only in the side chains attached to the outer ends of the tetrapyrrole 
nucleus. Descent with modification from a common ancestry seems clearly indi- 
cated for these photosynthetic species. 



106 • THE EVIDENCE FOR EVOLUTION 

The anthocyanins and anthoxanthins are water-soluble pigments found 
in the cell sap of plants, and are responsible for most of the flower and fruit 
colors in higher plants and for much of the color in autumn foliage. The antho- 
cyanins vary in color from red to purple to blue; the anthoxanthins, though 
chemically quite similar to the anthocyanins, appear yellow or white. The antho- 
cyanins are always combined with sugars to form glycosides, and the anthoxan- 
thins are usually found as glycosides also. The color, particularly of the antho- 
cyanins, changes with the acidity of the cell sap, becoming bluer as the acidity 
decreases. 





pelargonidin 
(anthocyanin; pink) 



apigenin 
(anthoxanthin; ivory) 



(aglycone 
residues) 



The anthocyanins and anthoxanthins of many hundreds of species of 
flowering plants have been studied both genetically and biochemically in one 
of the pioneer studies of biochemical genetics. The results have shown that these 
pigments are apparently derived from a common precursor and that the differ- 
ences among them are due to simple gene substitutions, which determine the 
state of oxidation and methoxylation of the side phenyl ring, the pH of the cell 
sap of the petals, and the position, number, and nature of the attached sugars. 
Such similarities, extending through many families of plants, certainly seem a 
strong argument for a common origin. 



Photoreceptors 

Even more remarkable, perhaps, are the biochemical homologies in- 
volved in photoreceptor systems, both animal and plant. Phototropism, photo- 
taxis, and vision are apparently all dependent on the yellow to red fat-soluble 
carotenoid pigments. The carotenes and the related xanthophylls are found in 
the chloroplasts, where their color is usually masked by the chlorophyll. Al- 
though relatively few studies have been made in plants or among the lower 
invertebrates, the available evidence implicates the carotenoids or their deriva- 
tives in the light reactions of these groups. Shown below is /3-carotene, the most 
familiar of the carotenoid pigments. 



COMPARATIVE BIOCHEMISTRY • 107 




H H CH 3 H H H CH 3 H H H H CH 3 H H H CH 3 H H 

I I I I I I I I I I I I I I I I I 

c=c-c=c-c=c-c=c-c=c-c=c-c=c-c=c-c=c 




/^-carotene 

The taxonomically intermediate position of the green flagellates such as 
Euglena, which have been claimed as algae by the botanists because they possess 
chloroplasts and as Protozoa by zoologists because of their other traits, is con- 
firmed by the presence of the carotenoid, astaxanthin, in the eyespot. Since this 
group contains both chlorophyll, a plant pigment, and astaxanthin, which is an 
exclusively animal carotenoid, it cannot properly be assigned to either the plant 
or the animal kingdom. 

The vertebrates and the higher invertebrates such as arthropods and mol- 
lusks cannot synthesize their carotenoids and must obtain them in their nutrition 
as the A vitamins, ultimately derived from plants. That the A vitamins are 
similar to the carotenes may be seen from the structure of vitamin A x . 




H H CH 3 H H H CH 3 H 

I I I I I I I I 

C = C-C = C-C=C-C = C-CH 2 0H 



vitamin Ai 



The carotenoid pigments play a fundamental role in photoreception in 
the arthropods, mollusks, and chordates. These phyla independently have devel- 
oped image-forming eyes, each of a distinct type, and yet each utilizes the A 
vitamins in the photoreception process. The details have been most carefully 
studied in the vertebrate eye. Photoreception takes place in the retina, where two 
types of photoreceptors are found: the rods, specialized for vision in dim light, 
and the cones, specialized for vision in bright light and for color vision. The 
action of light on the photosensitive carotenoid-protein pigments in these cells 
causes the carotenoid to split off from the protein, giving rise to nervous excita- 
tion, which is transmitted as a nervous impulse from the retina through the 
optic nerve to the brain where it gives rise to visual sensations. The chemistry 



108 • THE EVIDENCE FOR EVOLUTION 

has been most carefully worked out in the rods. Here the photosensitive pigment 
is rhodopsin, a rose-colored compound that is broken down by light through a 
series of steps to the protein, opsin, and to vitamin A 1 or its derivative, retinenej. 
The bleached products can regenerate rhodopsin spontaneously in the dark. 
Under continuous light the whole system goes into a steady state with the con- 
tinuous restitution of rhodopsin permitting vision to persist indefinitely. The 
phenomenon of dark adaptation, during which the ability to see in a dimly lit 
room markedly increases, can readily be explained as due to the resynthesis of 
rhodopsin, which was previously somewhat depleted in the light. The details of 
the changes in the rods are outlined in the diagram. (It may be noted that the 
rhodopsin is formed only from the so-called as optical configuration of retinenej 
but that it breaks down to the trans form. ) 




visual orange 

visual yellow 

\ 

cis retinene, + opsin : — ^ tr^nt retinene, + opsin (protein) 

u A ^ZZZI J t A 

as vitamin A x ~ trans vitamin A x 

(After Wald) 

The rhodopsin system utilizing vitamin A x is widely distributed, being 
found in the retinas of marine and terrestrial vertebrates. The crustaceans and 
the squid, a cephalopod mollusk, also use A t or retinenej in their visual pig- 
ments. However, the retina of fresh-water fishes contains a different light- 
sensitive pigment, a purple substance known as porphyropsin. The opsins are 
essentially the same as in rhodopsin, but the carotenoids are vitamin A 2 and 
retinene 2 , which differ from A 1 and retinenej in having just one extra double 
bond in the ring. This finding poses some very intriguing questions, for there 
are no fundamental phylogenetic distinctions between marine and fresh-water 
fishes; closely related species may be found in either environment. 

The available evidence indicates that the ancestral vertebrates lived in 
fresh water and had porphyropsin as their visual pigment. The evolution of the 
vertebrates gave rise to species that invaded the oceans or the land, and in both 
cases the invasion of the new habitat was accompanied by a shift from porphy- 
ropsin to rhodopsin. Study of the types intermediate in their habitats such as 
amphibians or fishes migrating between the sea and fresh water has shown that 



COMPARATIVE BIOCHEMISTRY • 109 



they also are intermediate in their visual pigments. These findings are sum- 
marized below. 



Marine fishes (Ai) 

\ 

Catadromous fishes (A^Ag) 
(e.g. eel) \ 

Anadromous fishes (A^A^ 
(e.g. salmon) V 

Fresh-water fishes t (A 2 )' 

Lampreys (A 2 ) 



Land vertebrates (A x ) 
Amphibians (A x and A 2 ) 





Crustacean eye 

(Au retinenei) 



Cephalopod eye 
(retinene x ) 



Invertebrate phototropisms 
(pigments unidentified) 



Green flagellate orientation 
(astaxanthin) 

Plant phototropism 
(carotene, xanthophyll) 



(After Wald) 



The type of pigment is not simply an adaptation directly determined 
by the environment, for one exceptional group of fish, the wrasse fishes 
(Labridae), is exclusively marine yet all have porphyropsin. Furthermore, the 
sea lamprey, which migrates from the ocean to fresh water to spawn, already has 
vitamin A 2 and porphyropsin as it starts its migration from the sea. Thus, genetic 
control of the type of visual pigment is clearly indicated. 

The lampreys are the most primitive living vertebrates and only dis- 
tantly related to the fresh-water bony fishes or teleosts. Hence, the presence of 
porphyropsin in this group places this type of pigment close to the origin of the 
vertebrate visual system. The lungfish, which have evolved along a separate line 
of descent from the modern fresh- water teleosts, also have vitamin A 2 in their 
retinas. 

Among the teleosts the salmon and the eels also migrate between the 
sea and fresh water. Migratory fish may be divided into two groups: anadromous, 
which migrate from the sea to fresh water to spawn, and catadromous, which 



110 • THE EVIDENCE FOR EVOLUTION 

migrate from fresh water to spawn in the sea. The retinas of anadromous salmon 
contain both rhodopsin and porphyropsin, vitamins A x and A 2 , but the porphy- 
ropsin predominates. The catadromous eels that return to the sea to spawn also 
have both pigments, with the rhodopsin predominant. Among all of the fish in 
these groups thus far studied, it has been found that their visual pigments are 
predominantly or exclusively the kind ordinarily associated with their spawning 
environment. 

The amphibians, which, as their name suggests, live on land or in the 
water or a little bit of both, are intermediate between a fresh-water and a ter- 
restrial existence. Their visual systems parallel their habitat, for those living in 
fresh water, such as tadpoles or the mud-puppy Necturus, a permanently larval 
aquatic form, contain vitamin A 2 , whereas terrestrial forms such as adult frogs 
have rhodopsin and vitamin A x . Even within a given species the type of visual 
pigment changes when metamorphosis makes possible a change in habitat. 

The vitamin Aj-retinenej-rhodopsin system appears to have originated 
somewhere in the evolutionary history of the invertebrates, and the vitamin A 2 - 
retinene 2 -porphyropsin system appears to be closely associated with the origin of 
the vertebrates. A major unanswered question is why a change from porphy- 
ropsin to rhodopsin should have taken place when fresh-water vertebrates 
evolved into marine or terrestrial species. The conclusion that porphyropsin con- 
fers an adaptive advantage in the fresh-water environment and rhodopsin is 
better suited to either an oceanic or terrestrial existence seems inescapable. The 
change from one system to another within the life cycle of a single individual 
seems the best indication that adaptation is involved. It must be remembered, 
however, that these changes are under genetic control and hence must have been 
brought about by natural selection and not by the direct influence of the environ- 
ment. 

Immunology 

Some unusual and valuable information about evolution has been de- 
rived from still another type of biochemical study — namely, immunology. The 
immunity of an organism is based upon what is called the antigen-antibody 
reaction. An antigen is a foreign substance of biological origin that is usually a 
protein although some polysaccharides are also antigenic. In response to the 
entrance of an antigen into the body, an antibody, which is a protein capable of 
combining specifically with that antigen, is formed. If the antigen subsequently 
enters the body again, the antibodies already present will combine with it, and 
the individual becomes immune to its harmful effects. Antibodies can be devel- 
oped not only against bacteria and viruses but against a variety of other sub- 
stances as well, and this fact has been utilized to study the relationships of 
organisms. 



COMPARATIVE BIOCHEMISTRY • 111 

If the blood serum or body fluid of an animal is injected into a rabbit, 
the rabbit forms antibodies in its blood against the foreign serum proteins. By 
withdrawing the rabbit's blood and removing the cells from the serum it is pos- 
sible to carry out the antigen-antibody reaction (foreign serum-rabbit antiserum) 
in a test tube, where a precipitate is formed. This so-called precipitin test or 
various refinements of it have been used in a number of phylogenetic studies, a 
few of which will be mentioned here. 

Some of the earliest studies were conducted by Nuttall. Perhaps the 
most exciting at the time was the discovery that rabbit serum containing anti- 
human antibodies reacted almost as strongly with chimpanzee serum as it did 
with human serum; somewhat less strongly with sera from the other apes; still 
less with monkey sera; only slightly with carnivore and ungulate sera; and essen- 
tially not at all with insectivore, rodent, and marsupial sera. Because of the spe- 
cificity of the antigen-antibody reaction these cross reactions are a measure of the 
degree of similarity of the serum proteins in the different species. They tend to 
confirm, therefore, the relationships of man to the Primates and particularly to 
the anthropoid apes. 

In another experiment Nuttall's group showed that the horseshoe crab, 
Limulus, once classified with the other crabs among the Crustacea, belonged in- 
stead much nearer the Arachnida, for an anti-Limulus serum reacted strongly 
with spider sera, but scarcely at all with crustacean sera. A more recent study by 
Wilhelm has shown a close serological relationship between echinoderms and 
hemichordates, which confirms the morphological evidence. Boyden has demon- 
strated that whales, which because of their adaptations to marine life were diffi- 
cult to place taxonomically among the mammals, are most closely related to the 
cloven-hoofed Artiodactyls. Another study by Moody indicated that rabbits and 
hares, long classed with the rodents, properly belong in the separate order 
Lagomorpha with closer affinities, actually, to the Artiodactyls than to the 
rodents. Thus, the serological approach has been very fruitful, particularly in 
instances in which the standard morphological methods were not too reliable. 



► SUMMARY 



The field of biochemistry has developed since Darwin's 
time to the point where it now can make notable contributions to 
our knowledge of evolution. Biochemical as well as structural 
homologies can be recognized, and they furnish reliable evidence 
of relationship independent of the conclusions based on compara- 
tive morphology. The chemical composition of living organisms, 
based on nucleic acids, proteins, carbohydrates, and fats, is itself 
evidence for the underlying kinship of all forms of life. Detailed 
studies of plant pigments, photoreceptor systems, immunology, 



112 • THE EVIDENCE FOR EVOLUTION 

and many metabolic systems have led to a variety of detailed bio- 
chemical evidence on relationships within and between groups. 
This evidence, unavailable to Darwin, has confirmed and extended 
our knowledge of evolution, for no other theory is adequate to 
interpret these data or so fruitful in suggesting further research in 
the field. 

SUGGESTED READING 

Boyden, A. A., 1942. "Systematic serology: a critical appreciation," Physiol. Zool., 
15:\09-W). 

, 1953. "Fifty years of systematic serology," Systematic Serol., 2:19. 

Florkin, M., 1949. Biochemical evolution (S. Morgulis, tr.). New York: Academic 
Press. 

Nuttall, G. H. F., 1904. Blood immunity and blood relationship. New York: Cam- 
bridge University Press. 

Prosser, C. L., I960. "Comparative physiology in relation to evolutionary theory," 
Evolution after Darwin, Vol. I, The evolution of life. S. Tax, ed. Chicago: 
University of Chicago Press. 

, ed., 1958. Physiological adaptation. Washington, D. C: American Physio- 
logical Society. 

Wald, G., 1952. Biochemical evolution. Modern trends in physiology and bio- 
chemistry. New York: Academic Press. 

, 1958. "The significance of vertebrate metamorphosis," Science, i28.T481- 

1490. 



CHAPTER 



12 



Biochemical Adaptation 



Biochemical as well as morphological adaptations can be 
discerned. The morphology of the animal in a sense simply re- 
flects its functioning; it is the net result of all of the genetic and 
environmental influences acting upon the developing organism. 
Regulation of the composition of the body fluids in different 
kinds of environments has led to a variety of biochemical adapta- 
tions. One of the fundamental similarities among living species 
of animals is in the relative ionic composition of the body fluids. 
Although they may differ in their absolute composition, neverthe- 
less on a relative basis the plasma of such diverse species as the 
jellyfish, lobster, frog, and man is quite similar, and furthermore 
is much like sea water (see Table 12-1). These similarities sug- 
gested to Macallum that the body fluids of animals were originally 
derived from sea water. Since it is widely believed that life origi- 
nated in the sea, the suggestion seemed quite reasonable. He even 
accounted for the discrepancies between the concentrations of 
potassium and magnesium in human plasma and sea water by the 
fact that the ocean millions of years ago contained less magnesium 
and more potassium than at present. The major difficulty with this 
theory is that it assumes that the body fluids, since being closed 
off from the sea, presumably at different times for different 
species, have somehow remained of the same composition despite 
the vicissitudes of existence and evolution in the history of each 
species. Since the evidence is clear that the ionic composition of 
the body fluids is actively maintained by living cells, the theory is 
obviously far too simple. An alternative explanation may be that 



113 



114 • THE EVIDENCE FOR EVOLUTION 

life can exist only within rather narrow limits and arose at a time when the ionic 
composition of the ancient seas was similar to that of the plasma of present-day 
animals. These ionic limitations have remained essentially unchanged; conse- 
quently, all subsequent evolution, no matter what direction it took, of necessity 
was accompanied by the development of mechanisms for maintaining the ionic 
composition of the body fluids within the limits that would support life. It is 
known that one of the requirements for life is enough water containing the 
proper concentrations of the right kinds of salts. 



TABLE 12-1 

Relative Ionic Compositions of the Bloods and "Tissue Fluids of 

Some Different Animals {After Macallum from Baldwin) 





Na 


K 


Ca 


Mg 


CI 


S0 3 


Sea water 


100 


3.61 


3.91 


12.1 


181 


20.9 


King crab 














Limulus 


100 


5.62 


4.06 


11.2 


187 


13.4 


Jellyfish 














Aurelia 


100 


5.18 


4.13 


11.4 


186 


13.2 


Lobster 














Homarus 


100 


3.73 


4.85 


1.72 


171 


6.7 


Dogfish 














Acanthias 


100 


4.61 


2.71 


2.46 


166 


— 


Sand shark 














Carcharias 


100 


5.75 


2.98 


2.76 


169 


— 


Cod 














Gadus 


100 


9.50 


3.93 


1.41 


150 


— 


Pollack 














Pollachius 


100 


4.33 


3.10 


1.46 


138 


— 


Frog 














Rana 


100 


— 


3.17 


0.79 


136 


— 


Dog 














Cants 


100 


6.62 


2.8 


0.76 


139 


— 


Man 














Homo 


100 


6.75 


3.10 


0.70 


129 


— 


Aquatic 


Life 













The maintenance of the proper concentration of salts is apparently a 
relatively simple matter for most marine animals. A word about osmosis is 
appropriate at this point. When two different solutions are separated by a semi- 
permeable membrane, which permits passage of the solvent but not of the dis- 
solved substances, the solvent will flow toward the solution of higher concentra- 
tion, thus tending to equalize the concentrations. This movement is known as 
osmosis or the osmotic flow, and the pressure resulting from this flow is osmotic 
pressure. Another way to think of osmotic pressure is as that amount of pressure 
necessary to prevent any fluid from flowing. A comparison of the freezing point 
of an aqueous solution with that of pure water serves as a simple yet precise 
indirect measure of the osmotic strength of that solution. In the coelenterates, 
echinoderms, and mollusks the freezing point depression of the body fluids does 



BIOCHEMICAL ADAPTATION • 115 

not differ essentially from that of the medium in which they live, and therefore 
their osmotic problems are not considered serious. However, the concentration of 
salts in fresh water is very low, and fresh-water animals have mechanisms for 
regulating their osmotic concentrations so that they are osmotically independent 
of their environments. Various methods have evolved in fresh-water species for 
osmotic regulation. Their problem, in essence, is to get rid of excess water. 
Semipermeable boundary membranes permit the retention of salts, but water is 
constantly seeping into the cells by osmosis, and must be eliminated in some 
way if the cells are not to swell up and burst due to the osmotic pressure. In the 
fresh-water protozoans contractile vacuoles constantly pump water out of the 
cell. Some protozoans can eliminate in this fashion a volume of water equal to 
their own volume in as little as two minutes. Species in other groups may have 
most of the body surface impermeable to both salts and water. The chitinous 
exoskeleton of crustaceans such as the crayfish, the keratin in the integument of 
various vertebrates, and the slimy surface of many fresh-water species all serve, 
to various degrees, to render the body surface impermeable. Excess water is still 
absorbed, but is eliminated by the excretion of a copious dilute urine through 
the kidneys of species such as the fresh-water bony fish and frogs. A frog, for 
example, excretes on the average one-third of its body weight in water each day. 
Man, with quite different osmotic problems, excretes only one-fiftieth of his 
weight per day. If the salt concentration is to be kept higher than that of the 
environment, osmotic work must be done in order to absorb salts against the 
concentration gradient. Fresh-water fish have special cells in the gills that carry 
out this function; mosquito larvae absorb chloride ions through their anal 
papillae. 

The marine teleosts or bony fishes, in contrast to the marine inverte- 
brates, have an osmotic concentration only about one-half as great as that of sea 
water. Dessication is therefore a constant threat, for they tend to lose water to 
their environment. With the Ancient Mariner, they can croak, "Water, water, 
everywhere, nor any drop to drink." Although they swallow large quantities of 
sea water, nevertheless their blood remains more dilute in salts than the sea 
water (see Table 12-2). The sea water is absorbed, salts and all, from the in- 
testine, but the excess salt is excreted by the so-called "chloride secretory cells" 
in the gills. Thus in both fresh-water and marine bony fish, osmotic regulation is 
achieved only by the expenditure of energy to do osmotic work in specially 
adapted cells in the gills. The salts move in opposite directions, of course, 
through the cells of these two groups. Whereas fresh-water teleosts excrete a 
copious dilute or hypotonic urine, marine teleosts waste a minimum of water, a 
valuable material to them, in the formation of urine, and their urine is nearly 
isotonic with the blood. The numerous glomeruli in the kidneys of fresh-water 
fishes appear to be adaptations for filtering off large amounts of water. Marine 
fishes, with the problem of conserving water, have few glomeruli and this region 



116 • THE EVIDENCE FOR EVOLUTION 

TABLE 1 2-2 

Freezing Point Depression of Body Fluids in Animals (°C) 
(After Heilbrunn) 



Marine animals 
Coelcnterata 

Alcyonium palmatum 
Echinodermata 

Asterias glacialis 
Annelida 

Sipunculus nudus 
Mollusca 

Ostrea edulis 

Octopus vulgaris 
Arthropoda 

Limulus polyphemus 

Homarus americanus 

Maja verrucosa 
Tunicata 

Ascidia mentula 
Chondrichthyes 

Mustellus vulgaris 

Raja undulata 
Teleostei 

Conger vulgaris 

Charax puntacco 

Fresh-water animals 
Mollusca 

Limnaea stagnalis 
Annelida 

Hirudo officinalis 
Crustacea 

Daphnia magna 

Telphus fluviatile 
Osteichthyes 

Cyprinus carpio 

Salmo jario 

Terrestrial animals 
Annelida 

Lumbricus terrestris 
Mollusca 

Helix aspera 
Insecta 

Decticus albifrons 

Lymantria dispar 

Bombyx mori 
Amphibia 

Rana esculenta 
Reptilia 

Emys europea 
Aves 

Chicken ? 
Mammalia 

Pig 

Horse 

Cat 



Body fluid 

2.195 

2.295 

2.27-2.31 

2.23 
2.16 

1.90 
1.82 
2.13 

2.08 

2.36 
1.89 

0.77 
1.04 

0.22-0.23 

0.43 

0.20-0.67 
1.17 

0.50 
0.57 

0.45-0.51 

0.37 

0.50 
0.48 
0.73-0.79 

0.40 

0.47 

0.615 

0.615 
0.564 
0.638 



Outer medium 


2.2 


2.195-2.36 


2.29 


2.11-2.14 
2.11-2.14 


1.82 
1.80 
2.17 


1.98 


2.29 
1.84 


2.14 
2.29 


0.02-0.03 



BIOCHEMICAL ADAPTATION • 117 

of the kidney has the appearance of having degenerated. This difference in the 
kidneys of marine and fresh-water species is also considered to be evidence for 
the fresh-water origin of the fishes. 

The marine elasmobranchs (sharks, skates, and rays) have about the 
same amount of salts in their blood as the marine teleosts, but they have in addi- 
tion about 2 percent urea (ordinarily a nitrogenous waste product) , which brings 
the total osmotic pressure to slightly higher than that of sea water. The solution 
of the osmotic problems posed by life in the sea is quite different, therefore, in 
teleosts and elasmobranchs. The urea is retained because the gills are relatively 
impermeable to urea in low concentrations, and the renal tubule contains a 
special segment that reabsorbs urea from the glomerular filtrate. The shark and 
its relatives resemble the fresh-water teleosts in certain respects, for its kidney is 
glomerular, the osmotic gradient tends to drive water into the fish, and the urea- 
absorbing segment corresponds to the salt-absorbing segment of the renal tubule 
in fresh-water bony fish. Certain elasmobranchs live in fresh waters, and it is 
believed that they are descended from forms that at one time lived in the sea 
and later invaded the rivers. The salt content in the plasma of marine and fresh- 
water elasmobranchs is almost the same, but the fresh-water species have only 
about 0.6 percent urea rather than 2 percent. Since a more copious dilute urine 
must be produced than even that of the fresh-water teleosts, it would appear 
advantageous if the urea content were further reduced or even eliminated en- 
tirely, but this is apparently impossible. During the long period of marine life, 
the physiology of the elasmobranchs became so completely adapted to the pres- 
ence of a high concentration of urea that the heart of fresh-water elasmobranchs 
will not beat in its absence. 

The presence of the glomerulus, a device for excreting water, is evi- 
dence to indicate that all of the fishes originated in fresh water. Invasion of the 
sea led to degeneration of glomeruli in the teleosts; in the elasmobranchs, the 
retention of urea furnished a different means of minimizing water loss. See 
Fig. 12-1. 

Terrestrial Life 

Life on land poses still other biochemical problems, for the environ- 
ment consists of air, with an abundance of oxygen but a scarcity of water. 
Furthermore, the excretion of nitrogenous waste products is more difficult in an 
environment where water is at a premium. The problems involved in biochemical 
adaptation to terrestrial life suggest that the first land vertebrates, the early 
amphibians, arose from among the fresh- water fishes rather than among the 
marine species living in the littoral zone. Two of the major adaptive changes 
required were the ability to obtain oxygen from the air rather than from water 
and the ability to withstand dessication. In warm, shallow, stagnant, fresh-water 



118 • THE EVIDENCE FOR EVOLUTION 

pools, the oxygen supply may be virtually depleted, and survival in this habitat 
may depend on the ability of the species to obtain the necessary oxygen from air 
rather than water. The air sac in fish is used as a lung by many species, particu- 
larly those dwelling in stagnant waters or in areas with seasonal droughts. The 
Dipnoi or lungfishes are perhaps the most familiar group of this kind, but the 
more primitive ray-finned fishes (Actinopterygii) such as the spoon-billed cat 
(Polyod on-Chondrostei) and the gar pike and bowfin (Lepisosteus and Amia- 



< 


cO 

LU 






CO 


1— 






UJ 


< 






UJ 


CO 


co 




> 

z 


UJ 

1— 


8 


CO 

1— 
co 


UJ 


> 


UJ 

—i 


U 


Z 


Z 


UJ 

1— 


i 

LU 


Qi 






\— 


< 


LU 




3 



< 2 

co < 

£ <"! 

< co^t 



" 






m 



i 



iiifiij 



^ 



SEA WATER 



n 



1 w 



RIVER WATER 



Fig. 12-1. Osmotic pressures of bloods of various animals compared 

with those of fresh and sea waters. A = freezing point depression. 

(After Baldwin.) 



Holostei) also use the air sac as a lung for getting oxygen from the air. The use 
of the air sac as a swim bladder or hydrostatic organ in the teleosts appears to 
have been a subsequent development in marine fishes. The modern lungfish 
Protopterus, during the seasonal drought in its habitat in Africa, estivates in a 
slimy cocoon, breathing by means of its lungs so that it is able to withstand 
dessication and obtain oxygen from air, the two requirements mentioned above. 
Furthermore, the fresh-water fish typically have an integument of low surface 
permeability to water, although water enters quite freely through the gill and 
oral membranes. Thus, in making the transition from fresh water to land, the 



BIOCHEMICAL ADAPTATION • 119 

problem is to control water loss at these points rather than over the entire body 
surface. 

It is doubtful that marine fishes were the first vertebrates to invade the 
land, since the littoral zone is a rather stable environment with an abundant 
oxygen supply and is therefore unlikely to require the major adaptive shifts that 
accompanied the origin of terrestrial vertebrates. Some of the most slowly evolv- 
ing groups, such as the oysters (Mollusca) and the horseshoe crab (Arthro- 
poda), inhabit the littoral zone, and their slow rate of evolution can probably 
be attributed to the stability of their environment and hence to the absence of 
major shifts in the pressures of natural selection that would be expected to 
produce rapid evolutionary change. 

Among terrestrial vertebrates water conservation is a major problem. 
In most of the amphibians, evaporation from the body surface occurs at a fairly 
rapid rate even though the skin is not completely permeable to the outward flow 
of water. No amphibian is altogether independent of a moist environment, for 
even the desert toads tend to burrow and seek out damp and humid places. The 
integuments of the reptiles, birds, and mammals are far more effective protection 
against surface evaporation, for their permeability to water is extremely low. The 
arthropods, the other major group of animals to have achieved virtually complete 
independence from a moist environment, are protected against surface evapora- 
tion by the chitinous exoskeleton. Both chitin and the cuticular wax contribute to 
the impermeability of the cuticle. 

Water loss during excretion is minimized in terrestrial forms in various 
ways. The ancestral vertebrates were fresh-water fishes whose kidneys primarily 
functioned, by means of large glomeruli, to rid the body of excess water. The 
frog kidney still functions in this fashion. In living reptiles, water loss has been 
reduced through a decrease in the size of the renal corpuscles, and consequently 
a smaller volume of filtrate is produced. In the snakes and lizards, the urine may 
even be solid or semisolid. The birds and mammals have renal corpuscles of 
normal size and therefore produce a large volume of filtrate, but the kidney 
tubule is modified by the presence of the long, thin loop of Henle in which it is 
thought most of the water resorption occurs. Some water is reabsorbed in any 
type of kidney tubule, but in man, for instance, with a long kidney tubule includ- 
ing the loop of Henle, scarcely 1 percent of the filtrate from the glomeruli ever 
reaches the bladder. The urine therefore is hypertonic to the blood in the birds 
and mammals. In birds, further water absorption occurs in the cloaca, and thus 
the urine becomes a semisolid mass. Insects, too, conserve water by reabsorption 
from the excretory wastes, which are discharged from the Malpighian tubules 
into the hind gut where resorption occurs. 

Terrestrial animals obtain water by drinking, or with their food, or as 
a product of metabolism. Absorption of water occurs in the small and large in- 



120 • THE EVIDENCE FOR EVOLUTION 

testine, and so the feces are usually semisolid or solid. The oxidation of organic 
compounds is a major source of water for some species, particularly desert 
species or such insects as clothes moths. The figures below indicate the efficiency 
of formation of metabolic water: 

Oxidation of 

100 g of G of water 
protein 41.3 

carbohydrate 55.5 

fat 107.1 

Thus the fats, which are frequently stored by desert mammals, produce almost 
twice as much metabolic water per gram oxidized as the other compounds. 

Development of amniote embryos on land is possible despite the fact 
that they are essentially aquatic. A watery environment is provided for reptilian 
and avian embryos by the shelled egg and for mammalian embryos by the uterus 
of the mother. Among the amphibians the majority of species lay their eggs in 
the water, and an aquatic larva, the tadpole, lives there for a considerable period. 
However, a variety of adaptations exist in various species of Amphibia for get- 
ting the eggs out of the water and minimizing the larval period. The reptilian 
egg may be regarded as the most successful of these adaptations. Viviparity is a 
further modification of reptilian development that has appeared not only in the 
mammals but also independently in certain reptilian groups as well. 

Nitrogen Excretion 

Nitrogenous wastes from protein metabolism are excreted in a variety 
of forms, with the type of waste product clearly related to the availability of 
water in the environment of the organism. Species with an abundant water 
supply excrete nitrogen primarily in the form of ammonia, a soluble but highly 
toxic compound. Although no group excretes just one nitrogenous waste product, 
the aquatic invertebrates and the fresh-water teleosts primarily eliminate am- 
monia, much of it through the gills in these teleosts rather than the kidneys. 
Marine teleosts, with quite a different osmotic problem as described earlier, 
excrete considerable ammonia, but they also excrete some urea and up to a third 
of their nitrogen as trimethylamine oxide, the latter two substances being soluble 
and relatively nontoxic. The elasmobranch fishes, which retain up to 2.5 percent 
urea in the blood, also excrete it from the gills. Terrestrial animals primarily 
excrete urea or else uric acid, which has a low toxicity and is relatively quite 
insoluble, hence can either be stored or eliminated as crystals. 

In frogs, the tadpoles eliminate 40 percent or more of their nitrogen as 
ammonia, but adult frogs, with a greater need for conservation of water, excrete 
less ammonia and about 80 percent urea. Salts and some water are reabsorbed in 



BIOCHEMICAL ADAPTATION -121 
NH 3 CH 3 

ammonia j NH 2 — C — NH 

CH 3 — N— CH 3 || 

II O 

O urea 

trimethylamine 
oxide 

H— N— 0=0 

I I 
0=C C— NH 

I II 
H— N— C— NH 
uric acid 



O 



the kidney tubules, and the evidence indicates that urea is actively secreted into 
the tubules. Mammals also excrete urea, during both embryonic and adult stages, 
and the urea may be concentrated up to 100 times its level in the blood by the 
reabsorption of water in the kidney tubules. 

Insects, birds, snakes, and lizards eliminate a semisolid urine containing 
uric acid crystals, thus minimizing water loss more than any other group. It 
should be noted that in these species with eggs protected against water loss 
(cleidoic eggs) the insoluble, nontoxic uric acid crystals can be stored in the 
allantois during the development of the embryo. 

Metamorphosis from a tadpole to a frog involves a number of dramatic 
morphological changes taking place in a relatively short time. As a result the 
organism changes from an aquatic gill-breathing herbivore to a terrestrial lung- 
breathing carnivorous tetrapod. Just as striking as the changes in structure are 
the biochemical changes that accompany metamorphosis. At that time nitrogen 
excretion shifts over primarily to urea from amomnia, the visual pigment changes 
from porphyropsin to rhodopsin, and the hemoglobin changes to a type with a 
decreased affinity for oxygen. It also has a declining affinity for oxygen as the 
acidity increases, the so-called Bohr effect. Tadpole hemoglobin exhibits no Bohr 
effect and has a relatively high affinity for oxygen. These three changes can be 
regarded as adaptive for terrestrial life although the evidence that this is so for 
rhodopsin is not yet available. They may also be considered as instances of bio- 
chemical recapitulation. The ancestors of the amphibians were fresh-water fishes, 
which excreted primarily ammonia, had porphyropsin in their retinas, and pos- 
sessed hemoglobin of high oxygen affinity and a small Bohr effect. It is difficult 
to avoid the conclusion that the developing frog manifests not only morpho- 
logical but biochemical recapitulation of a phylogenetic sequence. 

From this brief review, it seems clear that the biochemical approach to 
evolutionary problems and, conversely, the evolutionary approach to biochemical 
problems, are promising fields for further work, for this is an area of research 
where the surface has only been scratched. 



122 • THE EVIDENCE FOR EVOLUTION 

SUMMARY < 



The adaptations of living organisms to their environ- 
ments are biochemical in addition to being morphological and 
behavioral. Despite the varied osmotic problems posed by the sea, 
fresh water, and the land, living things must maintain the ionic 
composition of their body fluids within rather narrow limits. 
Water intake, water conservation, and the excretion of metabolic 
waste products are interrelated problems, the solutions of which 
vary greatly depending upon the environment. The invasion of 
fresh-water and terrestrial habitats became possible only when 
species had evolved methods of osmotic regulation in these new 
habitats. Evolutionary theories, therefore, must account for the 
origin of biochemical adaptation as well as the somewhat more 
obvious morphological adaptations. 

SUGGESTED READING 

Baldwin, E., 1949. Comparative biochemistry, 3d ed. New York: Cambridge Uni- 
versity Press. 

Prosser, C. L., and F. A. Brown, Jr., 1961. Comparative animal physiology, 2d ed. 
Philadelphia: Saunders. 

Smith, H. W., 1953. From fish to philosopher. Boston: Little, Brown. 



CHAPTER 



13 

Evolution in Animals 



Approximately a million species of animals have been 
described; in some groups such as birds and mammals virtually all 
species are known, but in others many more species undoubtedly 
remain to be discovered. The great number of living species prob- 
ably represents less than 1 percent of all of the species that have 
ever existed. These species have been arranged into a relatively 
small number of phyla, although there is no universal agreement 
among zoologists as to just how many phyla there are. The com- 
mon practice of arranging the different groups into a phylogenetic 
sequence is frequently a useful teaching device. The record is 
spotty, however, and its better known parts consist largely of 
modern species out at the tips of the evolutionary branches. Since 
the phylogenetically significant portions of the record may be 
obscured far in the distant past, too great stress on the phylo- 
genetic arrangement of known groups may confuse the student 
rather than convince him of the validity of the postulated rela- 
tionships. 

One of the problems in the discussion of evolution in 
the animal kingdom is the lack of familiarity of many people with 
the major groups of animals. This need not be an insurmountable 
obstacle. Most Americans can recognize at sight not only the 
make but the model and year of any car they spot on the highway. 
The number of phyla of animals is roughly comparable to the 
number of makes of American automobiles, and it should be no 
more difficult to learn to distinguish the phyla than it is to iden- 
tify cars. Furthermore, to remain unfamiliar with at least the 



123 



124 • THE EVIDENCE FOR EVOLUTION 

major animal groups is to be painfully ignorant of the world in which we live. 
Therefore, with no further apologies, we shall consider the major groups of 
animals and the ways in which they are thought to be related to one another. 
Obviously, many details must be omitted in our discussion, and if further in- 
formation about any of the groups is desired, the references at the end of this 
chapter should be consulted. 

A word or two may be in order about the nature of an animal. Anyone 
can tell the difference between a tree, which we call a plant, and a cow, which 
is an animal. The tree stands still and ignores you; the cow moves about, appears 
to see you, and may even, if so inclined, kick or toss or bite you. The tree makes 
its own food by photosynthesis from simple inorganic substances, but the cow 
cannot. However, not all animals can move, and not all plants are sessile, and 
distinctions based on behavior and nutrition soon begin to weaken. They break 
down completely in the flagellates or Mastigophora, which have traits regarded 
as characteristic of both animals and plants. The free-living flagellate, Euglena, 
is in many respects like an animal yet it contains chlorophyll and can therefore 
synthesize its own food. On the other hand, it can also absorb nutrients from its 
environment. It is not surprising that both botanists and zoologists have laid 
claim to such species, the botanists classifying them among the algae, the zoolo- 
gists among the Protozoa. The truth of the matter is that there is no sharp line 
of demarcation by which animals may be separated from plants. The living 
world is not divided into two camps, one plant, the other animal; rather, it 
forms a continuum. It is generally thought that the other Protozoa and the higher 
multicellular animals or Metazoa as well as the higher plants have arisen from 
ancestral primitive flagellates. 

Protozoa 

The Protozoa are fundamentally single-celled animals. Although some 
form colonies, nevertheless each cell is typically morphologically and physio- 
logically independent. (The Protozoa have also been called acellular animals 
because the high degree of complexity in some Protozoa outstrips anything to be 
seen in any individual metazoan cell. However, since the Metazoa seem to have 
been derived from the Protozoa, metazoan cells may perhaps best be thought of 
as having lost some of the versatility of the ancestral protozoan cell in their evo- 
lution to their present well-differentiated and specialized functions. The Protozoa 
do have a nucleus, cytoplasm, a plasma membrane, and the other structures usu- 
ally associated with cells; hence by the usual criteria it is difficult to avoid the 
conclusion that they are cells, highly versatile cells, but cells nevertheless.) The 
classification of the Protozoa into five classes based primarily on their mode of 
locomotion is as follows: 
1. Flagellata (Mastigophora) — propelled by one or several flagella. (A flagel- 

lum is a long whiplike cell process, often regarded as a very long 

mobile cilium.) 



EVOLUTION IN ANIMALS • 125 

2. Sarcodina (Rhizopoda) — amoeboid movement by means of pseudopodia 

(temporary protrusions of the protoplasm) . 

3. Sporozoa — all are internal parasites without locomotor organelles, usually 

producing spores. 

4. Ciliata — move by means of numerous cilia (short hairlike cell processes capa- 

ble of vibratory movement) . 

5. Suctoria — ciliated only in the young stages; as adults, have one or more 

suctorial tentacles. 

The relationships among the Protozoa are by no means clear, and their 
classification is to some extent quite arbitrary. Some of the green flagellates can 
hardly be separated from the green algae, and other flagellates, known as the 
chrysomonads, are continuous with the filamentous brown algae (Chrysophy- 
ceae). The chrysomonads show affinities in several directions; they may lose their 
flagella and resemble algae, or lose their chromoplasts and resemble animallike 
protomonads, or by the loss of both flagella and chromoplasts come to resemble 
typical amoebae or rhizopods. Loss of the chloroplasts in the different orders of 
flagellates has apparently given rise to the colorless animal forms. Furthermore, 
some parasitic flagellates with sporulation as a means of reproduction suggest the 
affinities of this group with the Sporozoa. The relationship between the flagel- 
lates and the Sarcodina is also suggested by the Rhizomastigina, which typically 
have both flagella and pseudopodia, as well as by the sporadic occurrence of 
amoeboid forms among various groups of flagellates. That the Sarcodina are 
derived from the flagellates rather than vice versa is suggested by the fact that 
they very often have flagellate immature stages, while the flagellates do not have 
amoeboid young stages. 

The flagellates may very well be a polyphyletic group — that is, derived 
from a number of different sources, in this instance, spirochaetes and bacteria, 
which in many cases also have flagella. The rhizopods, like the flagellates, also 
appear to have a polyphyletic origin from several different groups of flagellates. 
The origins of the Sporozoa are again somewhat of an enigma; possibly they are 
polyphyletic also. The ciliates and the suctorians are probably related, but their 
relations to the other protozoa are unclear although it has been suggested that 
the cilia are derived from flagella. 

Porifera 

The enormous diversity of form and function among the Protozoa, 
from the simplest amoeba to the most complex ciliate, is so great that the 
Protozoa are sometimes regarded as a subkingdom, separate from all of the 
multicellular animals or Metazoa. Among the multicellular animals the sponges 
or Porifera (pore bearers) are regarded as an evolutionary dead end from which 
no other groups have evolved. Therefore, they have been placed in a separate 



Deuterostomia 



Protostomia 
A. 




PORIFERA 




ENTOPROCTA 




? MESOZOA 



COELENTERATA 



Fig. 13-1. (facing and above). The phylogeny of the animal kingdom. 



128 • THE EVIDENCE FOR EVOLUTION 

branch of the Metazoa called the Parazoa. The sponges are rather simple sessile 
organisms, either asymmetrical or with radial symmetry. They have a cellular 
grade of construction with special cells for special functions. There are no 
organs, no mouth, and no nervous tissue. The body is permeated with pores and 
canals through which water currents flow. The currents are generated by the 
flagella of the collar cells or choanocytes that line the canals or chambers. Food 
particles are trapped by the collar cells and are digested intracellularly. There is 
an internal skeleton of spicules or of spongin fibers, which the Greeks used to 
line their helmets and which we use today to wash windows or automobiles. Be- 
cause of their characteristic choanocytes the sponges have been considered de- 
scended from the group of flagellates known as the choanoflagellates. However, 
it is also true that sponge larvae have typical flagellate cells rather than choano- 
cytes and hence the Porifera could have originated from some more generalized 
flagellate stock. The sponges have not evolved too far beyond the stage reached 
by colonial flagellates; although the cells are somewhat differentiated and spe- 
cialized for particular functions, coordinated activity has not been possible be- 
cause of the absence of any sort of a nervous system. Evolution in the sponges 
has led to increased complexity in the skeleton and in the system of water canals 
but not to any higher or more complex organisms. 

Mesozoa 

The phylogenetic position of the Mesozoa is not at all clear. One rea- 
son for this difficulty is th?t all of the species in the group are invertebrate 
parasites, and it cannot be said with certainty whether their simple structure is 
truly primitive or the result of the degenerative changes so frequent in parasites. 
The Mesozoa are small wormlike animals of extremely simple two-layered solid 
construction. Whereas the inner layer of the Metazoa is digestive in function, in 
the Mesozoa it consists of only one or a few reproductive cells. The outer layer 
of ciliated cells carries on intracellular digestion. This type of structure shows 
some resemblance to the ciliated planula larva of the coelenterates, and the 
Mesozoa have sometimes been treated with this group. In other cases they have 
been considered as degenerate flatworms. In view of the doubts about their 
origin and affinities it seems best to put them in a separate branch of the 
Metazoa. Until more evidence is available, however, it seems unwise to place too 
great emphasis on their phylogenetic importance as possibly the most primitive 
group of Metazoa. 

Coelenterata 

The Coelenterata (coel-enteron = hollow gut), which include such 
forms as corals, jellyfish, and sea anemones, have a gastrovascular or digestive 
cavity with a mouth but no anus, whence their name. They are tentacle-bearing, 



EVOLUTION IN ANIMALS • 129 

radially symmetrical Metazoa with a tissue level of construction. Their cells, un- 
like the Porifera, are organized into an outer protective epithelium or ectoderm 
and an inner digestive layer or endoderm. Though commonly called diploblastic 
(having two tissue layers), the coelenterates also have, to varying degrees, indi- 
cations in the mesogloea of a third intermediate mesodermal layer. Their activ- 
ities are coordinated by a nerve net so tHat food can be seized by the tentacles 
and brought to the mouth. Whether in the form of a sessile cylindrical polyp or 
a free-floating bell-shaped medusa or jellyfish, the tentacles typically bear sting- 
ing cells or nematocysts. 

Ctenophora 

The Ctenophora, the comb jellies or sea walnuts, are a small group of 
about 80 marine species; although frequently included in the Coelenterata, they 
are sufficiently distinct to warrant being placed in a separate phylum. They 
take their name, comb-bearing, from eight rows of ciliary combs used for loco- 
motion. Tentacles are present in most species, but nematocysts, so typical of 
coelenterates, are completely absent. Symmetry is biradial, a combination of 
radial and bilateral traits. They resemble the coelenterates in having a gastro- 
vascular cavity and in having essentially a tissue level of construction, but the 
presence of mesenchymal muscle fibers in the abundant mesogloea and of an 
aboral sensory region suggests a higher level of organization than that of the 
coelenterates. 

Platyhelminthes 

In the flatworms or Platyhelminthes, still greater complexity of organ- 
ization can be observed. The flatworms are bilaterally symmetrical; that is, they 
have anterior and posterior ends, dorsal and ventral surfaces, and right and left 
sides, one the mirror image of the other. Here there are clearly three germ layers 
with the mesoderm between the ectoderm and endoderm giving rise to muscles 
and other structures permitting greater complexity and efficiency. The flatworms 
have an organ level of construction, for their tissues are associated to form 
various organs. The excretory system is of the protonephridial type, consisting of 
terminal flame bulbs leading into excretory ducts. The flame bulbs lie in the 
body fluid and wastes diffuse across them into the ducts where a ciliary tuft (the 
"flame") presumably sets up a current in the duct. The nervous system has a 
pair of enlarged anterior ganglia and one to three pairs of longitudinal nerve 
cords. Hence, it is a central nervous system rather than a nerve net. Like the 
coelenterates, most of the flatworms have a gastrovascular cavity with a single 
opening that serves both as a mouth and anus. They completely lack any sort of 
body cavity comparable to the coelom of higher forms. Included in the Platy- 
helminthes are three quite distinct classes, the free-living flatworms such as 



130 • THE EVIDENCE FOR EVOLUTION 

Planaria of the class Turbellaria, the parasitic flukes or Trematoda, and the in- 
testinal parasites of vertebrates, the tapeworms or Cestoda. Associated with the 
parasitic habit, the parasitic flukes and tapeworms show varying degrees of 
change from the free-living turbellarians. 

Origin of the Metazoa 

While there is fairly wide agreement that the Porifera are derived from 
the choanoflagellates, the origin of the other Metazoa has been a moot question. 
A variety of possibilities has been raised, but no one theory can be said to have 
a preponderance of evidence in its favor. However, in a negative sense it is 
possible by a brief review of these theories to see which phyla are not likely to 
have been involved, and thus narrow the field considerably. The Metazoa sim- 
plest in structure are the Porifera, Mesozoa, Coelenterata, Ctenophora, and the 
Platyhelminthes. We have already considered and more or less discarded the 
Porifera and Mesozoa, which leaves the other three phyla. Of these, the co- 
elenterates and the flatworms are the two groups most commonly considered as 
lying closest to the original Metazoa. It should be realized that the fossil record 
has been of no help in settling the question of the origin of the Metazoa, for 
the presence of most of the major phyla among the fossils of the early Paleozoic, 
when the record first becomes fairly good, indicates that the Metazoa must have 
arisen well back in the Pre-Cambrian. Therefore, the various theories are pri- 
marily speculative and all could very well be wrong. The following theories are 
among the more prominent concepts thus far advanced. 

1. The gastraea theory of Haeckel may be regarded as the classical 
theory of metazoan origin, certainly it is the most widely quoted. In its current 
form, colonial flagellates similar to Volvox, which forms a hollow, spherical 
colony, are equated with the hollow spherical blastula stage in the embryology 
of the Metazoa. This hypothetical organism, termed the blastaea, was supposed 
to have a single layer of flagellated cells and to swim about with one end always 
forward so that an antero-posterior axis was established. The first differentiation 
was assumed to be into somatic or body cells and reproductive cells, a phe- 
nomenon also observed in Volvox. Next the posterior cells of the blastaea were 
thought to become adapted or specialized for digestive functions,, the assumption 
being that separation of the digestive and locomotor functions would have an 
adaptive advantage. If one side of the sphere is pushed inward or invaginated — 
as can be done with a deflated basketball, for example — so that it comes in 
contact with the other side, a pouchlike, two-layered, radially symmetrical struc- 
ture is formed that approaches the basic structure of the coelenterates. It also has 
the form of the two-layered or diploblastic gastrula stage of the metazoan embryo 
— whence the name, gastraea, of this hypothetical organism. 



EVOLUTION IN ANIMALS • 131 

The two-layered coelenterate ancestors were then supposed to have 
given rise to the flatworms by becoming bilaterally symmetrical and developing 
a third germ layer, the mesoderm, between the outer ectoderm and the endo- 
derm. The small ciliated planula larva of the coelenterates has been compared 
with the ancestral type that gave rise to the bilaterally symmetrical flatworms 
presumably like the very simple ciliated free-living flatworms of the order Acoela 
of the class Turbellaria. The appeal of the theory lies in its synthesis of a great 
deal of information drawn from the embryology and morphology of existing 
forms. In fact, it might be said that it is almost too good to be true. For example, 
the origin of the internal digestive layer, or endoderm, in lower forms, is gen- 
erally not by invagination but rather through the inward migration of many 
cells from the ectoderm, and the planula larva and acoeloid flatworms have an 
internal solid mass of cells rather than being hollow. Other criticisms have also 
been directed at the theory as outlined above, but it seems likely that it will re- 
main a strong contender for some time to come. 

2. Another suggestion is that the coelenterates, like the sponges, are 
off the main path of metazoan evolution and arose independently of the rest of 
the Metazoa. The flatworms then would become ancestral to the higher Metazoa. 
However, the presence of a gastrovascular cavity in both coelenterates and flat- 
worms and of a mesogloea between the ectoderm and the endoderm of the 
coelenterates comparable to the mesoderm of the flatworms suggests a relation- 
ship between them. Furthermore, the Ctenophora, while not necessarily in a 
direct line of relationship between the two groups, appear to show some similar- 
ities to both. 

3. Still another hypothesis is that the coelenterates have evolved from 
the flatworms rather than vice versa as in the gastraea theory. In this case multi- 
nuclear ciliates were postulated to give rise to the Turbellaria Acoela by the 
formation of cells around the nuclei. From the Acoela were descended the 
higher Turbellaria from which the higher invertebrates arose and from which 
the coelenterates and the ctenophores were separately and independently evolved. 
On this view bilateral symmetry was the primitive condition, and the radial sym- 
metry of the coelenterates was a secondary development associated with their 
sessile mode of life. 

4. Quite a different concept is that the Metazoa, except for their mode 
of nutrition, are more like multicellular plants than like Protozoa and that the 
earliest organisms were multinuclear and photosynthetic plants, which were 
ancestral to the Metazoa and, independently, to the flagellates and the other 
Protozoa. 

Although other theories or other versions of the above theories have 
been advanced, these give some idea of the diversity of opinion on the subject. 
The concept followed in the phylogenetic chart in Fig. 13-1 is that of the 



132 • THE EVIDENCE FOR EVOLUTION 

planula-acoela line of descent, not only because it is currently perhaps the most 
highly regarded of the various possibilities but also because it is less of a strain 
on the imagination. One reason is that the transition from radial to bilateral 
symmetry can be more readily visualized. This change was a major one, leading 
to the evolution of the higher phyla, all of which are bilateral. As noted above, 
however, since it cannot even be stated with assurance that the change was in 
this direction, further emphasis on the origin of bilaterality seems unwarranted. 
However the stage of the primitive acoeloid flatworms may have been reached, a 
stage similar to this seems very likely to have been ancestral to the higher bi- 
lateral groups. Although again all of the relationships among the various phyla 
cannot be discerned, two major lines of descent can be recognized: one, the 
Protostomia, leading to the Arthopoda and Mollusca; the other, the Deutero- 
stomia, leading to the Chordata. The distinction between the Protostomia and 
the Deuterostomia is based on their mode of development. In the Protostomia, 
the mouth forms from (or in the region of) the blastopore whereas in the 
Deuterostomia the anus forms from (or in the region of) the blastopore, and 
the mouth is formed de novo. In the Protostomia, furthermore, embryonic de- 
velopment typically proceeds by spiral cleavage and is determinate; that is, spe- 
cific cells of the early embryo are fated to give rise to specific parts of the larva 
and their extirpation results in a deficient larva. The trochophore larva character- 
istic of this group, more or less spherical in shape, has an apical tuft of cilia, a 
ciliated band (the prototroch) at the equator, and a complete L-shaped digestive 
tract. 

Nemertea 

The flatworms were mentioned earlier as lacking a coelom or body 
cavity, and one other phylum, the Nemertea (also known as Nemertinea and 
Rhynchocoela) or ribbon worms, is also acoelomate. They resemble the flatworms 
in several respects, having, for example, a ciliated ectoderm and flame bulbs for 
excretion. They differ, however, in having a complete digestive tract with mouth 
and anus, an eversible proboscis not connected with the alimentary canal, and a 
simple blood vascular system, differences so fundamental that assignment to a 
separate phylum seems necessary. 

Acanthocephala 

A fairly large number of groups have a body cavity known as a pseudo- 
coel, since it lacks the mesodermal lining characteristic of the coelom. The spiny- 
headed worms or Acanthocephala are parasitic as larvae in various arthropods 
and as adults in the intestine of vertebrates. Though having a pseudocoel and 
circular as well as longitudinal muscles, they entirely lack a digestive tract, the 



EVOLUTION IN ANIMALS • 133 

retractable proboscis serving as an organ of attachment and the food being 
directly absorbed from the host's intestine. The excretory organs appear to be 
nephridia with modified flame bulbs, and in some, a type of superficial seg- 
mentation appears. Although these traits in general resemble those of the other 
pseudocoelomates such as the nematodes, the embryology tends to resemble that 
of the flatworms. Therefore, the Acanthocephala, even though a small group, 
have generally been accorded the status of a separate phylum. 

The next six groups of pseudocoelomate animals to be considered show 
many similarities and therefore have sometimes been placed in one phylum, the 
Aschelminthes. These groups, which here are treated as separate phyla, are the 
Nematomorpha (Gordiacea) or horsehair worms, the Priapulida, the Kinor- 
hyncha (Echinodera), the Nematoda (Nemathelminthes) or roundworms, the 
Gastrotricha, and the Rotifera. These more or less wormlike animals all have a 
complete digestive tract with a posterior anus. 

Nematoda 

Of these six groups, the nematodes include by far the largest number of 
species, for there are literally thousands of free-living and parasitic forms, some 
of an extremely unusual nature; one species, for example, has been found only 
in the poison gland of the rattlesnake. A roundworm is a rather simply con- 
structed animal. In addition to the traits noted above, the body is covered by a tough 
cuticle, and the body wall has only a single layer of longitudinal muscle cells. 
There are no respiratory or circulatory organs, and the excretory system, when 
present, is a simple canal system unlike that of any other phylum. The nervous 
system consists of a circumenteric ring around the pharynx and a simple system 
of associated ganglia and nerves. 

Nematomorpha, Kinorhyncha, and Priapulida 

The Nematomorpha are much like the nematodes except that no excre- 
tory system is present, the alimentary canal is always more or less degenerate, 
and there is just a single ventral nerve cord. The long, thin adults, thought to 
resemble "horsehair," are free-living, but the larvae are insect parasites. Another 
small group, the Kinorhyncha (Echinodera), are superficially segmented into 
13 or 14 rings and have a retractable spiny anterior end. There are two excretory 
tubes or protonephridia each with a single flame bulb. The Priapulida, with only 
three known species, are also superficially segmented, but have circular as well as 
longitudinal muscles. The spiny retractile anterior end calls to mind the kinor- 
hynchs, as does the type of nervous system. The soft posterior processes with 
gill-like outgrowths seem to be unique. The excretory system consists of proto- 
nephridia and solenocytes (similar to flame bulbs except that they have a single 



134 • THE EVIDENCE FOR EVOLUTION 

flagellum rather than a tuft of cilia). Although the priapulids have been grouped 
with the sipunculid and the echiurid worms either in a separate phylum Gephy- 
rea or else as a class of annelids, this seems clearly in error, for their greatest 
affinities are with the kinorhynchs and nematodes, and they also show certain 
traits similar to those of the rotifers and gastrotrichs. 

Gastrotricha and Rotifera 

Typical gastrotrichs are minute spiny animals that glide about by means 
of ventral cilia. Each lobe of the forked posterior end has an adhesive gland for 
temporary attachment. The excretory system consists of paired protonephridia 
each with a single flame bulb. The rotifers have a similar excretory system, an 
anterior retractile ciliated disc or corona, and a posterior forked "foot" with 
adhesive glands. The internal jaws in the pharynx are unique and quite distinc- 
tive. The rotifers are generally the smallest of all of the Metazoa. 

The gastrotrichs are probably closest phylogenetically to the nematodes, 
but they also have several features in common with the rotifers, such as external 
cilia, the forked foot, and the excretory system. The rotifers, because of their 
resemblance to the trochophore larva characteristic of the annelid-mollusk line 
of descent, are thought to be in some way related to the common ancestor of 
these phyla. However, the rotifers also resemble the free-living flatworms, per- 
haps more than they do any other group, as well as showing affinities with the 
gastrotrichs and nematodes. Hence they should probably be regarded as a group 
relating the turbellarian flatworms to the aschelminths. 

Entoprocta and Ectoprocta 

The final pseudocoelomate phylum, the Entoprocta, was formerly 
placed with the Ectoprocta as a class in the phylum Bryozoa (or Polyzoa), but 
the resemblance is superficial. The entoprocts have a pseudocoelom, a U-shaped 
digestive tract with both mouth and anus opening within the circle of tentacles, 
and they have protonephridia with flame bulbs for excretion. The ectoprocts, a 
much larger group, have a true coelom lined with mesoderm, an anus that opens 
outside the lophophore bearing ciliated tentacles, and no excretory organs. The 
similarities lie primarily in the crown of tentacles and the sessile mode of life, 
which is usually in colonies. However, since the tentacular crown of the ento- 
procts is not comparable or homologous to the lophophore of the ectoprocts, it 
is clear that the two groups should be separated. The group nearest the ento- 
procts would seem to be the rotifers. Despite the many well-defined differences 
between adult entoprocts and ectoprocts, both types develop from a type of larva 
known as the trochophore, although the entoproct larva departs in some respects 
from the typical trochophore larva. 



EVOLUTION IN ANIMALS '135 

Among the animals with a pseudocoel, then, are six groups quite clearly 
similar and two phyla, the Acanthocephala and the Entoprocta, rather different 
from the others. Here, too, although fundamental morphological similarities exist 
that clearly seem to indicate relationship, the exact phylogenetic sequence is ob- 
scured in the mists of the past and may never be known with certainty. 

Brachiopoda and Phoronida 

In addition to the Ectoprocta, two other coelomate phyla, the Brach- 
iopoda and the Phoronida, also have a lophophore, and these three phyla, though 
quite different in some respects, nevertheless appear to be related. They are 
similar also in having a trochophore-like larva but differ in that both phoronids 
and brachiopods have a simple circulatory system and an excretory system with 
nephridia, both of which are lacking in the ectoprocts. The nephridial system of 
coelomate invertebrates is typically of the metanephridial type, in which the 
nephridial tubules begin as coelomic openings, draining wastes from the body 
cavity. 

Very few species of phoronids are known. Sedentary, wormlike animals, 
they are all marine, living in a self-secreted tube from which the lophophore is 
extended to feed. The brachiopods or lamp-shells have a superficial resemblance 
to the bivalve mollusks such as the oyster, but the two halves of the shell are 
dorsal and ventral rather than right and left halves as in the bivalves. An unusual 
feature of brachiopod development is the formation of the mesoderm by entero- 
coely (out-pocketing from the gut) , a mode of mesoderm formation more char- 
acteristic of the Deuterostomia and therefore suggesting affinities with the echino- 
derms and chordates. The brachiopods have a long, extensive fossil record, and 
the living species represent only a small remnant of the species and genera of 
the past. One living genus, Lingula, has persisted virtually unchanged from the 
Ordovician period of the Paleozoic, some 400,000,000 years ago, and is therefore 
probably the oldest living genus. 

Mollusca 

The Mollusca are the second largest group of invertebrates, having five 
classes, quite diverse in appearance but with an underlying fundamental similar- 
ity. The body consists of a head (absent in bivalves and tooth shells), a ventral 
muscular foot, and a dorsal visceral mass covered by a mantle, which usually 
secretes a calcareous shell on its upper surface. The five classes are as follows : 

1. Amphineura — chitons 

2. Gastropoda — snails, slugs, limpets, whelks, abalone, periwinkle, conches, etc. 

3. Scaphopoda — tooth shells 

4. Pelecypoda — bivalves such as clams, oysters, scallops, and mussels 

5. Cephalopoda — nautili, squids, and octopi 



136 • THE EVIDENCE FOR EVOLUTION 

The radula, a rasping organ in the mouth of most mollusks, is unique to the 
group, and here, for the first time, we encounter respiratory organs either in the 
form of gills (ctenidia) or lungs. Though mollusks are coelomate, the coelom is 
reduced to the cavities of the gonads, the pericardium, and the nephridia. Both 
circulatory and excretory systems are well developed. The nervous system varies 
widely from the simple system of ganglia in bivalves like the clam to the com- 
plex centralized system with a "brain" and camera-type eyes of cephalopods such 
as the squid. 

The mollusks were a large, well-defined group with all of the living 
classes already represented at the beginning of the Paleozoic. The trochophore 
larva typical of many mollusks clearly indicates their relationship to the line of 
descent that also led to the annelids and arthropods, although the separation 
must have occurred long ago. Most of the Mollusca show little or no evidence 
of segmentation, and the group is usually referred to as unsegmented. However, 
the recent discovery of a living mollusk, Neopilina galatheae, in the depths off 
the west coast of Mexico has raised serious questions as to whether the ancestral 
mollusks were segmented (see Fig. 13-2). Neopilina belongs to the Amphineura, 
generally presumed to be closest to the ancestral mollusks because of their rela- 
tively simple bilateral structure as compared with the other classes of mollusks. 
Neopilina has five pairs of small gills, and each gill is associated with a 
nephridium; there are, furthermore, five pairs of dorso-ventral muscles associated 
with the foot. Clearly, this arrangement represents well-defined segmentation, 
and the possibility must now be admitted that ancestral mollusks were seg- 
mented, the modern forms representing a secondary loss of the segmented con- 
dition. If such is the case, then the mollusks may be closer to the annelids than 
had been previously suspected. 

Annelida 

The members of the phylum Annelida, to which belong the earthworms, 
polychaete marine worms, and leeches, are usually conspicuously segmented both 
externally and internally, with the body composed of many essentially similar 
segments or somites. This segmentation can be observed not only in the append- 
ages and muscles, but in the serial repetition of the parts of the nervous, excre- 
tory, circulatory, and reproductive systems. Each somite also typically bears small 
rodlike appendages or setae. The circulatory system consists of a closed system of 
vessels with a circulating fluid containing a respiratory pigment. The larva, when 
present, is a trochophore, and the early development of annelids and mollusks is 
quite similar. 

Since segmentation is present in the two dominant phyla of animals of 
the present time, the Arthropoda and the Chordata, it must represent a major 
evolutionary advance. However, although various theories of the origin of seg- 



EVOLUTION IN ANIMALS • 137 



Fig. 13-2. Neopilina galatheae, a 
recently discovered living mollusk 
of the class Amphineura, with 
definite signs of segmentation, 
suggesting a closer relationship 
between the mollusks and the seg- 
mented annelids than had previ- 
ously been suspected. (With per- 
mission of Lemche.) 




mentation have been advanced, there is little evidence to favor any one theory 
over the rest Furthermore, segmentation in the annelid-arthropod line appears to 
have arisen independently of segmentation in the chordates. 

Sipunculida and Echiurida 

The sipunculid and echiurid marine worms are undoubtedly related to 
the annelids and, perhaps because they are rather small groups, have sometimes 
been classified as annelids. Since they are quite different from the earthworm and 
other annelids, however, more recently they have been placed in separate phyla. 
Both Sipunculida and Echiurida have trochophore larvae, large coeloms, and 
somewhat similar circulatory and nephridial systems. The sipunculid or "peanut" 
worms are gourd-shaped with a narrow retractile anterior end crowned with a 
circle of ciliated tentacles. The anus is anterior and dorsal. The echiurids have a 
troughlike proboscis, which cannot be withdrawn into the anterior end of the 
body like that of the sipunculids, and the anus is posterior. Bristlelike setae are 
present, and the larvae show definite signs of segmentation. Thus the echiurids 
quite definitely belong close to the annelids. 



Onycophora, a Living Link 

Whereas the evidence for the relationships among the various groups 
presented thus far has been rather tenuous in most cases, the evidence for the 
relationship between annelids and arthropods is much more clearcut. These 
phyla show many similarities both in mode of development (although a tro- 
chophore larva is absent in arthropods) and in adult structure. The arthropods 



138 • THE EVIDENCE FOR EVOLUTION 

differ from annelids in having a thick chitinous exoskeleton, jointed appendages, 
and muscles in functional groups rather than simple continuous sheets. The 
coelom of the arthropods is much reduced and is more or less replaced by the 
haemocoele of the circulatory system, and the excretory and reproductive systems 
are concentrated rather than segmental. 




Fig. 13-3. Peripatus (Macroperipatus geayi) of the phylum Onycophora, a 
connecting link between the annelids and the arthropods. (Photo by Ralph Buchs- 

baum.) 



These two phyla are the only major invertebrate groups with con- 
spicuous true segmentation. One other small phylum, the Onycophora, is also 
segmented, and has a unique mixture of annelid and arthropod traits. They are 
like annelids in having segmental nephridia, simple eyes but no well-defined 
head, a soft cuticle, short un jointed appendages, and muscles in continuous 
sheets. Arthropod traits include the reduced coelom with the haemocoele as the 
adult body cavity, the tracheal respiratory system, and the circulatory system with 
a dorsal "heart." The Onycophora, represented by Peripatus (Fig. 13-3), have 
been classed with the arthropods and also as annelids, but it seems best to place 
them for the present in a separate phylum, for their features, although re- 



EVOLUTION IN ANIMALS • 139 

sembling those in both groups, are different enough to suggest that the Onyco- 
phora are a very old group. Rather than being a missing link between Annelida 
and Arthropoda, they perhaps represent a third independent line of descent from 
the ancestral stock that gave rise to modern annelids and arthropods. In any 
event their very existence tends to reinforce the postulated relationship between 
those two phyla. 

Arthropoda 

The Arthropoda have by far the greatest number of species of any 
phylum. The following classes have been recognized, most of them including 
very familiar forms. 

1. Trilobita — extinct aquatic forms 

2. Crustacea — shrimps, copepods, crabs, lobsters, etc. 

3. Arachnida — spiders, ticks, mites, scorpions, horseshoe crabs, eurypterids (ex- 

tinct), etc. 

4. Myriapoda — centipedes, millipedes 

5. Insecta — butterflies, beetles, bees, dragonflies, etc. 

The Arthropoda may be described as segmented animals with jointed append- 
ages, a haemocoele, and a thick chitinous exoskeleton. This body plan has been 
enormously successful in all sorts of habitats. Different species have adapted to 
life in the depths of the sea, on land, and in the air. The exoskeleton undoubt- 
edly made possible the invasion of the land by protecting the animals against 
dessication, and, by providing rigid points of attachment for the muscles, it also 
is related to their speed of movement. Furthermore, the great morphological 
specialization and diversification of the exoskeleton into various types of legs, 
wings, and mouth parts has made possible adaptation to a great variety of eco- 
logical niches. 

Chaetognatha and Pogonophora 

The phyla remaining to be considered, Chaetognatha, Echinodermata, 
Pogonophora, Hemichordata, and Chordata, all belong to the Deuterostomia. 
The arrow worms or Chaetognatha resemble in the simplicity of their structure 
(no excretory, respiratory or circulatory systems) some of the pseudocoelomate 
groups. However, they have a large true coelom and their early embryology re- 
sembles that of the echinoderms and chordates. A post-anal tail is found only in 
this group and among the chordates. The bristles about the mouth, from which 
the phylum gets its name, aid in the capture of food. Although the arrow worms 
appear to belong among the Deuterostomia, they show no obvious relation to 
any other members of this group. The Pogonophora, sedentary worms living in 



140 • THE EVIDENCE FOR EVOLUTION 

long tubes in the depths of the Pacific, were originally thought to be polychaete 
annelids, but more recently they have been placed in a separate phylum with 
their closest affinities to the Hemichordata. Because of the complex tentacles at 
the anterior end, somewhat like the lophophore of the phoronids, ectoprocts, 
and brachiopods, they have been placed between the hemichordates and the 
lophophorates. However, the exact status of this group will not be well estab- 
lished until it has been more extensively studied. 

Echinodermata 

The Echinodermata, which include such species as starfish, crinoids, 
brittle stars, sea urchins, and sea cucumbers, have ciliated, free-swimming, bi- 
laterally symmetrical larvae and radially symmetrical adults, presumably a sec- 
ondary development related to the adults' sessile mode of life. Although a star- 
fish is a far cry from a vertebrate, nevertheless the echinoderms, hemichordates, 
and chordates clearly form a related group. The relationship is based primarily 
on the similarities in their embryological development. In the Deuterostomia not 
only is the mouth newly formed, the blastopore becoming the anus, but cleavage 
is indeterminate, and the mesoderm and the coelom originate from pouches 
formed from the wall of the primitive gut (enterocoely) . Furthermore, the 
echinoderm skeleton is derived from the mesoderm as it is in the chordates, un- 
like its mode of origin in any other invertebrate group. The different groups of 
echinoderms have several distinctive types of larvae, but in the early stages of 
development all echinoderm larvae pass through a dipleurula stage during which 
they show several traits in common. The dipleurula larvae are bilaterally sym- 
metrical, swim by means of longitudinal looped ciliated bands, and have an 
anterior coelom that opens to the dorsal surface through a pore. There is an 
anterior tuft of sensory cilia, a ventral mouth, and a posterior anus. The develop- 
ing Hemichordata pass through stages very similar to the dipleurula larva, and 
the tornaria larvae of the hemichordate tongue worms are so similar to the 
bipinnaria larvae of the starfishes that they were originally described as starfish 
larvae (Fig. 13-4). These larvae and their mode of development are so different 
from the trochophore larva characteristic of the mollusk-annelid line that the 
larval traits have served as the basis for the diphyletic system of evolution de- 
scribed here. Although larval resemblances and differences may be misleading 
because the larvae themselves may evolve in adapting to their environments, the 
differences between dipleurula and trochophore larvae appear to be more funda- 
mental than can be accounted for by differing adaptive responses. Finally, it 
should be noted that the larvae of these and other forms are best interpreted as 
recapitulating the larvae of the ancestral forms rather than as being representa- 
tive of the adult ancestor. 

Adult echinoderms have unsegmented bodies usually with five arms (or 
multiples of five) bearing tube feet. The water vascular system, of which the 



EVOLUTION IN ANIMALS • 141 



Mouth 




Adult ACORN WORM 
Fig. 13-4. Larval homology in the echinoderms and the hemichordates. 

tube feet form a part, is a unique system for locomotion, respiration, and food 
handling. The digestive system is complete though the anus is small (in some it 
is lacking), and the coelom is well developed. Nervous and circulatory systems, 
though present, are reduced. Among all the invertebrates, the starfish and its kin 
seem very unlikely candidates as relatives to the phylum that we, at least, tend 
to regard so highly, the chordates. 

Hemichordata 

The hemichordates have sometimes been classified as a subphylum of 
the Chordata, but more recently the trend has been to call them a separate 
phylum. Small wormlike animals, they have indications of the three chordate 



142 • THE EVIDENCE FOR EVOLUTION 

traits — notochord, pharyngeal gill slits, and dorsal nerve cord — but in each case 
some doubt exists as to their homology. The body is composed of a proboscis, a 
collar, and a trunk, each region having separate coelomic cavities. The mouth 
opens at the anterior margin of the collar into the digestive tract, and just back 
of the collar numerous gill slits permit excess water to pass out of the tract. 
There is some question as to whether the gill slits have a respiratory function. The 
"notochord" or stomocord projects forward into the proboscis as an anterior out- 
pocketing of the digestive tract and serves as a supporting structure, but whether 
it is truly homologous to the notochord is doubtful. Since the ventral nerve cord 
is more extensive than the dorsal one that is limited to the collar, again the 
homologies are not clear. Thus, although the acorn worms are clearly more like 
the chordates than like any other group, they are still sufficiently different to be 
considered as a separate phylum. 

An extinct group known as the graptolites has recently been included 
among the Hemichordata, but the evidence for this relationship is rather tenu- 
ous, and further information seems necessary before any well-founded conclu- 
sions can be drawn. 

Chordata 

The phylum Chordata has three subphyla: 

1. Urochordata or Tunicata — the tunicates or sea squirts or ascidians 

2. Cephalochordata — amphioxus or the lancelets 

3. Vertebrata — the back-boned animals or vertebrates 

The sessile adult tunicate shows little to suggest its affinity to the other chordates, 
but the free-living larvae clearly show chordate characteristics. The notochord of 
the larva is confined to the tail (whence the name Urochordata). The dorsal 
hollow nerve cord terminates anteriorly in a "brain" and a median eye. Gill 
slits are found in a region comparable to the pharynx in the higher chordates, 
and thus all three traits are clearly present. Upon settling down, the larva has its 
tail reabsorbed, the notochord disappears, and the nervous system is reduced to 
a ganglion. The gill slits are incorporated into a large branchial sac, and a test 
or tunic is secreted over the outer surface. 

In the cephalochordates the three distinctive chordate traits are seen in 
simple form in the adults. The notochord and dorsal nerve cord extend the 
length of the body up into the anterior tip (hence the name Cephalochordata, 
even though they have no distinct head). Numerous gill arches associated with 
the circulatory system are found in the pharyngeal region. Amphioxus is a fre- 
quent subject of study in zoology, for the circulatory, muscular, nervous, and 
other systems are thought to be representative of the ancestral chordate condi- 
tion. The presence of nephridia that appear to resemble those of certain poly- 
chaete annelid worms constitutes something of a phylogenetic puzzle. 



EVOLUTION IN ANIMALS • 143 

The vertebrates, whose evolution has already been discussed, are the 
dominant animals on the earth at present, for to this group belong the fishes of 
the sea, the mammals on the land, and the birds of the air. Although many 
species as adults lack gills and a notochord, nevertheless at some stage in the 
life cycle the basic chordate traits appear and the relationship of all of these 
groups to one another is clearly evident. 

Confronted by the great diversity of species, one well can wonder 
whether it is possible to decipher any sort of orderly relationship among so many 
thousands of kinds of animals. The surprising thing perhaps is not that so few 
well-defined relationships have been pinned down, but rather that the phylogeny 
of the animal kingdom is as well known as it is. When the great gaps in our 
knowledge of the past are realized, it is easier to appreciate the problems in- 
volved. One other factor that is almost impossible for the human mind to en- 
compass is the vast stretch of time available in the past during which some of the 
otherwise almost unbelievable evolutionary changes took place. If the magnitude 
of the evolutionary changes of just the past ten million years can be appreciated, 
it becomes perhaps somewhat easier to comprehend the magnitude of changes 
possible during periods ranging up to hundreds of millions of years. 

— ► SUMMARY 

Any survey of the animal kingdom tends to stress the 
means of distinguishing the different kinds of animals from one 
another, but it must be remembered that all animals share many 
traits in common. Furthermore, despite many questionable or 
dubious points, it is possible to work out a phylogeny of the 
animal kingdom based on the similarities among the different 
groups. Although such a phylogeny is based on the assumption of 
evolution, the very fact that the phylogeny, when constructed, 
forms a branching system is in itself an argument favoring 
evolution. 

SUGGESTED READING 

Berrill, N. J., 1955. The origin of vertebrates. Oxford: Clarendon Press. 

Borradaile, L. A., and F. A. Potts, 1958. The Invertebrata, 3d ed. New York: 
Macmillan. 

Buchsbaum, R., 1948. Animals without backbones, 2d ed. Chicago: University of 
Chicago Press. 

de Beer, G. R., 1954. The evolution of metazoa. Evolution as a process. J. Huxley, 
A. C. Hardy, and E. B. Ford, eds. London: Allen and Unwin. 

Hyman, L. H., 1940-1959. The invertebrates, Vols. 1-5. New York: McGraw-Hill. 

Marcus, E., 1958. "On the evolution of animal phyla," Quart. Rev. Biol., 33/24-58. 

Storer, T. I., and R. L. Usinger, 1957. General zoology, 3d ed. New York: McGraw- 
Hill. 

Young, J. Z., 1950. The life of vertebrates. Oxford: Clarendon Press. 



CHAPTER 



U 

Evolution in Plants 



In the plant kingdom as in the animal kingdom classi- 
fication has been attempted in a way that conforms with the 
postulated phylogeny of the various groups. This effort has been 
only partially successful, for again in many cases the relationships 
are difficult to decipher and arbitrary decisions have been neces- 
sary. However, because additional research seemed to indicate that 
the existing classification did not accurately reflect the relation- 
ships among the various plants, a major revision in classification 
of the plant kingdom was recently made. The classical classifica- 
tion was as follows: 

Kingdom Plantae 

Division Thallophyta 

Subdivision Algae — seaweeds, kelps, pond scum, etc. 
Subdivision Fungi — molds, yeasts, bacteria, mushrooms, etc. 
Division Bryophyta 

Class Hepaticae — liverworts 
Class Musci — mosses 
Division Pteridophyta 

Class Filicineae — ferns 
Class Equisetineae — horsetails 
Class Lycopodineae — club mosses 
Division Spermatophyta 

Subdivision Gymnospermae — conifers 
Subdivision Angiospermae — flowering plants 
Class Dicotyledoneae 
Class Monocotyledoneae 



144 



EVOLUTION IN PLANTS • 145 

The more modern classification, which has been based on recent mor- 
phological and paleobotanical work and is believed to be a more natural system, 
is as follows: 



formerly 
Algae 



formerly 
Fungi 



formerly 
Pteridophyta 

formerly 
Pteridophyta 

formerly 
Spermatophyta 



Kingdom Plantae 

Phylum* Cyanophyta — blue-green algae 

Phylum Euglenophyta — euglenoids 

Phylum Chlorophyta — green algae 

Phylum Chrysophyta — yellow-green and golden brown 

algae and diatoms 
Phylum Pyrrophyta — cryptomonads and dinoflagellates 
Phylum Phaeophyta — brown algae 
Phylum Rhodophyta — red algae 

Phylum Schizomycophyta — bacteria 
Phylum Myxomycophyta — slime molds 
Phylum Eumycophyta — true fungi 

Phylum Bryophyta — mosses, liverworts, and hornworts 

Phylum Tracheophyta — vascular plants 

SUBPHYLUM PSILOPSIDA 

Subphylum Lycopsida — club mosses 
Subphylum Sphenopsida — horsetails 

Subphylum Pteropsida 
Class Filicineae — ferns 

Class Gymnospermae — conifers 
Class Angiospermae — flowering plants 



The major changes can be seen to be an upgrading in the systematic 
rank of the various algae, reflecting the belief that these groups are not at all 
closely related, and a rearrangement in the classification of the different groups 
of higher plants. The latter change seemed necessary because recent evidence has 
tended to break down some of the former distinctions between the pteridophytes 
and the spermatophytes. 

The terms thallophyte, algae, and fungi are, however, useful ones and 
undoubtedly will continue to be used even though it is recognized that they 
represent artificial groupings. The phyla considered as thallophytes are plants 
that lack true roots, stems, and leaves (or to be more specific, the vascular tissues, 
xylem and phloem), and in which the zygote does not form a multicellular 



* The Botanical Rules of Nomenclature recognize "Divisions" rather than "Phyla," but 
the latter term is used here to parallel zoological usage. 



146 • THE EVIDENCE FOR EVOLUTION 

embryo while still in the female sex organs. The algae are thallophytes possessing 
chlorophyll; the fungi are thallophytes lacking chlorophyll. The postulated rela- 
tionships among the different plant phyla are shown in Fig. 14-1. 

Cyanophyta 

The phylum Cyanophyta (or blue-green algae) is an extremely primi- 
tive group. The plant is a single cell, occasionally grouped in loose aggregations. 
There apparently is no definite nucleus, for the chromatin appears scattered in 
the center of the cell. The chlorophyll is diffused rather than being organized 
into plastids. The blue color is due to another pigment, phycocyanin, and a red 
pigment may also be present. The only known method of reproduction is by 
asexual fission, and none of the cells of the blue-greens has flagella. The Cyano- 
phyta have been described from Precambrian rocks estimated to be a billion years 
old and are, therefore, among the oldest known fossil plants. 

Rhodophyta 

The phylum Rhodophyta (or red algae) takes its name from the red 
pigment phycoerythrin associated in the plastids with chlorophyll and also in 
some species with phycocyanin. The thallus is ordinarily multicellular, composed 
of nucleated cells. The life cycle may be complex, with both sexual and asexual 
reproduction, but an unusual feature of these algae is the absence of any type of 
flagellated reproductive cell. The red algae have a fossil record going back to the 
Ordovician and show little resemblance to any other algal group except the blue- 
greens. Both groups lack flagellated cells and have in common, in at least some 
species of both groups, the red and blue pigments, phycoerythrin and phy- 
cocyanin. 

Pyrrophyta and Chrysophyta 

The cryptomonads and dinoflagellates have been placed by botanists in 
the phylum Pyrrophyta. Most members of this phylum are unicellular with two 
unlike flagella, yellow-green to golden-brown plastids, no cell walls, and reserve 
food in the form of starches or oils. 

The Chrysophyta include the yellow-green algae, the golden brown 
algae, and the diatoms. The name chrysos, "golden," stems from the fact that 
there are more yellow or brown carotenoid pigments than there is chlorophyll, 
with both pigments being found in plastids. The food reserves are oils and 
leucosin, an insoluble carbohydrate. The cell walls are usually formed of over- 
lapping halves, frequently silica impregnated. The three classes of this phylum, 
in some ways quite different, are thought to be related because of the similar 




METAZOA 

Other 

PROTOZOA 



SCHIZOMYCOPh 
(Bacteria) 



©4© ® 



CYANOPHYTA 
(Blue-green algae) 



F*#. 24-1. The phylogeny of the plant kingdom. 



148 • THE EVIDENCE FOR EVOLUTION 

types of reserve food and the silicified bipartite cell walls. The Pyrrophyta and 
Chrysophyta show some affinities, but the phylogenetic relationships of these two 
groups are still far from clear. 

Phaeophyta 

The brown algae or Phaeophyta have their photosynthetic pigments 
masked by the brown pigment, fucoxanthin. The plants are multicellular, ranging 
in size from a few cells to the giant kelps over 100 feet in length, and are 
vegetatively the most highly specialized group among all of the algae. Not only 
may the plant bodies be highly differentiated, but a variety of methods of repro- 
duction have evolved, and there is commonly an alternation of generations. 
Although the brown algae have become the most advanced in structure among 
the algae, resembling in some respects the primitive vascular plants, they are not 
thought to have given rise to any higher groups of plants nor are they considered 
to be very closely related to any other group of algae. 

Euglenophyta and Chlorophyta 

Almost all of the Euglenophyta are naked unicellular flagellates with 
the chlorophyll not associated with any other pigments except the usual caroti- 
noids (carotene and xanthophyll) found in the green algae and the higher 
plants. They differ from the blue-green algae in having the reserve food in the 
form of the carbohydrate, paramylum, and fats. 

The green algae or Chlorophyta have chlorophyll and the associated 
carotenoids in the same proportions as the higher plants. The cells have definite 
nuclei and chloroplasts, are often flagellated, and the thallus may be unicellular, 
multicellular, or colonial. The reserve food is starch, and cellulose cell walls are 
present; in these respects the green algae differ from the euglenoids. However, 
the green algae are clearly rather similar to the euglenophytes and are thought 
to have been derived from them. Furthermore, both the bryophytes and the vas- 
cular plants are considered to have evolved from filamentous green algae. 

Schizomycophyta 

Although the bacteria (Schizomycophyta) show some structural and 
reproductive similarities to the blue-green algae and to some of the true fungi, 
their exact phylogenetic position is unknown and will probably remain a matter 
of speculation. They are extremely small (up to 5 microns) and structurally 
simple unicellular organisms. Bacteria are generally believed to have been among 
the first living organisms on earth. Most bacteria are parasites or saprophytes 
(obtaining food from nonliving organic matter), and are called heterotrophic. 
However, some bacteria, such as iron and sulfur bacteria, are autotrophic — that 



EVOLUTION IN PLANTS • 149 

is, capable of synthesizing organic compounds from simple inorganic substances. 
Some of the rich iron ore deposits of the earth are extremely old and are thought 
to have been formed by the action of iron bacteria, which obtain the necessary 
energy for organic syntheses from the oxidation of ferrous compounds in iron- 
bearing waters. Thus, these autotrophic chemosynthetic bacteria could have ex- 
isted even before the photosynthetic process had evolved. Furthermore, since 
evidence is accumulating as to ways in which organic compounds could have 
been synthesized by nonliving systems under different environmental conditions 
in the distant past, it is conceivable that heterotrophic bacteria could also have 
preceded photosynthetic organisms. Some bacteria are photosynthetic, and the 
bacteria have been suggested as possible progenitors for both the algae and the 
fungi. However, this hypothesis is by no means well established, and it has also 
been suggested that the three groups have evolved in parallel from an unknown 
common ancestor or even that the bacteria are a degenerate rather than a primi- 
tive group. The latter hypothesis seems to have less evidence in its favor, and 
the current tendency is to regard the bacteria as truly primitive plants, but their 
exact relationships to other microorganisms and plant groups are likely to remain 
obscure. 

Myxomycophyta and Eumycophyta 

The slime molds or Myxomycophyta are typically saprophytes with an 
unusual life cycle that includes both animal and plantlike features. The organism 
consists of a naked multinucleate protoplasmic mass or plasmodium, which 
creeps slowly about in an amoeboid fashion and is capable of ingesting solid 
food particles. Under favorable conditions, the plasmodium ceases to move and 
forms spore-bearing fruiting bodies or sporangia, characteristic of plants. The 
affinities of the slime molds are uncertain, for they appear to be transitional 
forms between the plant and animal kingdoms. In some respects they seem more 
closely related to certain protozoa than to any other groups, yet they also show 
similarities to the more primitive true fungi or Eumycophyta. 

The true fungi are quite a diverse group. Common to all of the Eumy- 
cophyta is their heterotrophic nutrition and their ability to produce spores, and 
most of them have plant bodies consisting of masses of filaments or hyphae. 
Three suggestions have been made as to the origin of the true fungi. They show 
some resemblance to the Myxomycophyta, to certain Protozoa, and also to some 
of the algae, from which they might have arisen through loss of chlorophyll. 
However, again the exact phylogeny is unknown. 

Overlapping Systems of Classification 

At this point it may be well to stop and reassess some of the material 
just covered, for there is a fundamental inconsistency that needs to be brought 
out in further detail. The systems of classification for the plant and animal king- 



150 • THE EVIDENCE FOR EVOLUTION 

doms that have been outlined above are rather generally used and are widely 
accepted by botanists and by zoologists. However, in some respects, these group- 
ings into plant and animal phyla are deceptively clear-cut. For example, most 
zoologists classify the groups known as cryptomonads, chrysomonads, phyto- 
monads, chloromonads, euglenoids, and dinoflagellates in the class Flagellata of 
the phylum Protozoa. Most botanists, on the other hand, regard cryptomonads 
and dinoflagellates as members of the algal phylum Pyrrophyta, chrysomonads as 
members of the phylum Chrysophyta, euglenoids as Euglenophyta, and phyto- 
monads (or Volvocales) and chloromonads as Chlorophyta. Furthermore, some 
zoologists consider the slime molds, which botanists classify as the phylum 
Myxomycophyta, to be an order, the Mycetozoa, of the class Sarcodina (or 
Rhizopoda) of the phylum Protozoa. These differences are not altogether the 
result of chauvinistic tendencies of the two groups of scientists, but rather reflect 
the fact that it is virtually impossible to draw a well-defined line between ani- 
mals and plants. Clearly, since the zone of overlap is so broad, all living things 
belong to one great interrelated system, and the separation into plant and animal 
kingdoms must be regarded as a convenient but artificial device. 

Another approach to this problem has been the creation of a third 
kingdom, the Protista, in addition to Animalia and Plantae. Included in the 
Protista are such groups as the bacteria, the protozoa, and the slime molds. 
Although this system has some merit, in that some of the duplication can be 
avoided, it has the drawback that two artificial lines are required rather than one. 
However, it is to be hoped that in time the historical barriers between botany and 
zoology will gradually erode, and a generally accepted biological system of classi- 
fication for the lower organisms will emerge, which will lack some of the diffi- 
culties of the system now in use. At the present time there is no generally ac- 
cepted system of classification covering all living things. Although this discovery 
may be disconcerting to the beginning biology student who likes to have things 
neatly packaged with no loose ends, to the student of evolution it should come 
as no surprise, for it tends to confirm the validity of the theory of evolution. 

No mention of the phylogenetic position of the viruses has been made 
thus far, simply because there is virtually nothing to say. The viruses consist of 
the hereditary material, DNA (deoxyribonucleic acid, or in some cases RNA, 
ribonucleic acid) covered by a protein sheath, and are so simple in structure that 
it has been impossible to relate them to any other living group. Indeed, the ques- 
tion of whether they can properly be called "living," since they can be crystal- 
lized, has even been raised. Here, too, as with the bacteria, it has been suggested 
that they are degenerate rather than primitive. 

Bryophyta 

The so-called higher plants are now placed in the subkingdom Em- 
bryophyta and have the following traits in common: terrestrial plants, multi- 
cellular embryos that are retained in the female sex organs, and an alternation of 



EVOLUTION IN PLANTS -151 

a multicellular gametophyte generation with a multicellular sporophyte genera- 
tion. Both phyla in the Embryophyta — that is, the Bryophyta (mosses, liver- 
worts, and hornworts) and the Tracheophyta (vascular plants) — are thought to 
be descended from the green algae (Chlorophyta) . The category Embryophyta, 
like Thallophyta, is an artificial one because the bryophytes and the vascular 
plants appear to have originated independently from the green algae. Although 
it was formerly believed that the bryophytes gave rise to the vascular plants, the 
first fossils of vascular plants come from Devonian and Silurian deposits whereas 
fossil bryophytes have not been found until millions of years later in the Car- 
boniferous. Thus, the present belief is that the bryophytes appear to represent an 
evolutionary dead end because they became adapted, without complete success, to 
terrestrial life, but have never given rise to any further better adapted groups of 
plants. 

The bryophytes are small in size, lack true roots, stems, and leaves as 
well as vascular tissue (xylem and phloem), and have a rather small sporophyte 
generation that is dependent or parasitic on the larger, independent gametophyte 
to which it remains attached. They depend on water for fertilization, since the 
motile sperm swim to the egg, and in this they can be compared to the Amphibia, 
a group that also has become largely terrestrial but in which breeding still ordi- 
narily must take, place in the water. In fact, only the gymnosperms and angio- 
sperms do not require "environmental" water for fertilization. 

Tracheophyta 

In contrast to the bryophytes, the sporophyte is the predominant inde- 
pendent generation in the tracheophyte life cycle. The Tracheophyta or vascular 
plants are characterized by the presence of some type of tracheary element and a 
vascular system made up of xylem and phloem, and all are land plants except a 
few that have secondarily returned to water. In the tracheophytes the root system 
is adapted for the absorption of water and salts that are transported to the shoot 
system, which is adapted for photosynthesis. The manufactured food is carried 
throughout the plant by the vascular system. The shoot, exposed to the air, is 
protected against water loss by a cuticle, but openings or stomata permit the ex- 
change of gases with the atmosphere. 

Origin of Vascular Plants 

The exact origin of the vascular plants is still a mystery, but they are 
now generally thought to have been derived from the green algae through the 
differentiation of the thallus into root and shoot. The discovery of a very ancient 
order of fossil plants, the Psilophytales, has tended to support this theory, for 
they are of extremely simple structure and can be thought of as a group, yet 
various members show indications of having given rise separately to the Lycop- 



152 • THE EVIDENCE FOR EVOLUTION 

sida (club mosses or ground pines), the Sphenopsida (horsetails), and the 
Pteropsida (ferns, conifers, and flowering plants). See Fig. 14-2. 

Some of the Psilophytales resemble algae because they have dichotomous 
branching but no leaves or roots. However, they differ in having a cuticle, 
stomata, a vascular system, and cutinized spores. Furthermore, certain psilophytes 
have very small leaves suggesting the club mosses, while others indicate leaf 
formation of a different type, by the flattening and broadening of the branch 
system. In this case the leaves are comparable to those of broad-leaved plants 
such as the ferns. Still another type shows the whorled pattern characteristic of 
the horsetails. Thus, within this one group are found fossil types suggestive of 
all of the other subphyla of vascular plants. The subphylum Psilopsida, well rep- 
resented as fossils in the Silurian and Devonian some 350 to 380 million years 
ago, are now represented by just two genera of the order Psilotales. 

The Lycopsida are another group that appear to have had their heyday 
in the Paleozoic and have persisted in a few genera as a relatively insignificant 
part of the present-day flora. In the Carboniferous, the coal that was formed 
came from the remains of these and other plants. Their leaves are structurally 
simple and spirally arranged, branching is dichotomous, and unlike the psilopsids 
they have distinct roots, stems, and leaves. 

The horsetails, like the Lycopsida, arose in the Devonian, flourished in 
the Carboniferous, and have since dwindled into insignificance. Perhaps their 
most striking character is the arrangement of the small leaves in whorls, but they 
also have roots and jointed stems. 

The dominant living plants belong to the Pteropsida. Of these, the ferns 
appear to be the oldest group and are thought to have given rise to the seed 
plants. The ancient ferns, along with the horsetails and the club mosses, formed 
the dominant vegetation of the Carboniferous. The ferns also appear to have 
evolved directly from the Psilophytales. 

The gymnosperms, to which the conifers belong, seem to have evolved 
from the ferns through the seed ferns (Cycadofilicales), fossil seed plants with 
many fern like traits. All of the gymnosperms are woody plants with naked 
seeds. 

The angiosperms or flowering plants, which are dominant in the present 
flora, present a complete mystery with respect to their origin. They are generally 
considered to have evolved from one of the groups of gymnosperms, but even 
though the Cycadofilicales, the Bennettitales, the Gnetales, and the Caytoniales 
have all been suggested as progenitors of the angiosperms, there is no reliable 
evidence at present in support of any one of these gymnosperm groups or of any 
other. The fossil record is of little help, for many fossils of flowering plants are 
found in Cretaceous deposits, but no older, possibly transitional forms have yet 
been discovered. Within the angiosperms, it is thought that the Ranales (butter- 
cups and magnolias) are the most primitive. These plants belong to the dicoty- 



EVOLUTION IN PLANTS • 153 




<c ^v — mr^ 




B 





D 



Fig. 14-2. Representatives of the primitive order of vascular plants, the 
Psilophytales. A, Rhynia — simple member of group. B, Asteroxylon — possi- 
bly related to the ancestors of the Lycopsida (the club mosses). C, Hyenia — 
possibly related to the ancestors of the Sphenopsida (the horse tails). 
D, Pseudosporochnus — possibly related to the ancestors of the Pteropsida 
(the broad-leafed plants). (With permission of Fuller and Tippo.) 



154 • THE EVIDENCE FOR EVOLUTION 

ledons (mustards, poppies, roses, peas, composites, etc.), which have two seed 
leaves serving as storage organs for food. The monocotyledons (grasses, lilies, 
palms, etc.) used to be considered more primitive but are now thought to have 
been derived from the dicots. 

In plants as in animals, many phylogenetic questions remain to be 
answered. Although it is not unreasonable to suppose that answers will be found 
to some — for example, the origin of the angiosperms — on the other hand, com- 
pletely satisfactory answers to others may never be forthcoming. However, new 
discoveries continue to be made and new insights gained, so that in time the rela- 
tionships among living things will be much better understood than they are at 
present. 



SUMMARY < 



The classification of the plant kingdom has recently been 
rather extensively revised. This revision was designed to bring 
the system into better accord with current thought on phylo- 
genetic relationships among plants. The general effect has been to 
separate the algae into distinct phyla, thus emphasizing the differ- 
ences among them, while grouping the higher vascular plants into 
a single phylum, Tracheophyta. Studies in paleobotany as well as 
plant anatomy are making the history of evolution within the 
plant kingdom increasingly well understood. Although many de- 
tails remain to be learned, the record, even as it stands, is a clear- 
cut case for evolution. 

SUGGESTED READING 

Arnold, C. A., 1947. An introduction to paleobotany. New York: McGraw-Hill. 

Axelrod, D. I., I960. "The evolution of flowering plants," Evolution after Darwin. 
Vol. I, The evolution of life. S. Tax, ed. Chicago: University of Chicago 
Press. 

Bold, H. C, 1957. Morphology of plants. New York: Harper. 

Fuller, H. J., and O. Tippo, 1954. College botany, 2d ed. New York: Holt. 

Stebbins, G. L., 1950. Variation and evolution in plants. New York: Columbia Uni- 
versity Press. 

Walton, J., 1953. An introduction to the study of fossil plants. London: Black. 



CHAPTER 



15 

Genetic Evidence 



Hybridization 



A matched team of mules is a sight rapidly passing from 
the American scene. The proverbial stubbornness and hardiness 
of the mule are no doubt responsible for developing the equally 
renowned vocabulary of the muleskinner. To the question, "What 
is a mule?" several answers can be given in addition to what a 
muleskinner might have to say about their character and person- 
ality. A mule is a species hybrid, the offspring of a jackass (Equus 
asmus) and a mare (Equus cabalius), and as such is a prime 
example of hybrid vigor or heterosis, a phenomenon frequently 
observed in the progeny of two genetically dissimilar individuals 
(see Fig. 15-1). A mule is also an evolutionary dead end, for 
with very rare exceptions mules are sterile. By their very existence 
mules pose the question, "Why can two clearly distinct species 
hybridize?" and still another, "Since they can form viable, vigor- 
ous offspring, why are these offspring sterile?" The answers to 
the enigma of the mule are wrapped up in the theory of evolu- 
tion. The hereditary material of the two species is quite evidently 
sufficiently similar for fertilization to occur and for normal devel- 
opment to proceed under the joint control of the genes from both 
species. The formation of normal gametes (or sperm and egg 
cells) requires, however, the pairing of similar or homologous 
chromosomes. Since the chromosomes of these two species differ 
in both number and composition, normal pairing or synapsis can- 
not take place. From that point on, normal gamete formation is 
disrupted. The interpretation is that these species trace back to a 



155 



156 • THE EVIDENCE FOR EVOLUTION 




Equus ? 

Fig. 15-1. The existence of the mule, sterile offspring of the cross between mare 
and jack, is readily explained by the theory of evolution. These two species, 
descended from a common equine ancestry, are still enough alike genetically to 
produce a viable hybrid when crossed, but their chromosomal and genetic differ- 
ences are too great to permit normal meiosis and gamete formation in the mule. 



common ancestor in the not too distant past, and that their genetic mate- 
rials are still sufficiently similar to permit normal fertilization and develop- 
ment. However, during the course of evolution their chromosomes and genes 
have diverged to such a degree that they no longer are enough alike to permit 
normal gamete formation. Other theories leave unanswered the question of why 
hybridization is possible at all in two clearly distinct species such as these. 

Man has attempted many other crosses between different species, and 
long lists have been compiled of the results of these crosses, many of which have 
been successful. In general, the greater the similarity between the species, the 
greater the likelihood of success in hybridizing them. Each successful cross raises 
once again the question of why such crosses are possible if each species had a 
separate, independent origin. 



GENETIC EVIDENCE • 157 

In addition to the artificial hybrids many naturally occurring hybrids 
have also been observed, especially in plants. Relatively little work has been done 
with the nonvascular plants — algae, fungi, mosses, etc. — but in vascular plants, 
hybridization has been found with unexpected frequency in a variety of different 
groups. Natural hybrids have been reported in ferns and in a number of genera 
of conifers or gymnosperms such as pine, juniper, and fir. Among the flowering 
plants or angiosperms the number of known natural hybrids continues to increase 
as further study brings to light more and more instances of hybridization. Some 
groups of woody plants such as the oaks and willows seem especially likely to 
form natural hybrid swarms. Certain other groups have been so disrupted by 
natural hybridization and its after-effects that their taxonomy is almost a hopeless 
mess. Among them are the blackberries (Rubus), the hawthorns (Crataegus) , 
the dandelions (Taraxacum), the hawkweeds (Hieracium) , and many genera of 
grasses. 

Though less common than in plants, natural hybrids in animals are by 
no means unknown. Among the invertebrates only a few phyla have been care- 
fully studied for natural hybrids. However, despite rather careful study in the 
insects, relatively few natural hybrids have been reported, the best known cases 
being among the crickets and the butterflies and moths. Among the vertebrates 
quite a number of natural hybrids have been reported in the fresh-water fishes 
such as the sunfish, suckers, and trout. Hybrid swarms of toads of the genus 
Bufo are examples from the amphibians, and quite a few hybrids between dif- 
ferent species of birds, particularly the ducks, have been recorded. Hybridization 
in the reptiles and in the mammals is apparently quite rare. It seems probable 
that ethological or "psychological" isolation, not a factor in plants, contributes in 
a significant way to the rarity of natural hybrids in animals. However, these few 
examples should suffice to show that even without man's intervention, hybridiza- 
tion does occur in both plants and animals. The theory of evolution gives a rea- 
sonable explanation for this capability. 

Not only have many casual or accidental hybrids been reported, but 
many species of plants have actually arisen subsequent to hybridization. Amphi- 
ploidy (also called allopolyploidy; a more detailed discussion of polyploidy will 
be given later) is the result of the doubling of the chromosome number of a 
sterile, interspecific hybrid and gives rise to a stable, fertile, true-breeding new 
species at a single step. It is one of the commonest ways in which new species of 
plants have arisen, and approximately a third of the species of flowering plants 
are estimated to have originated in this manner. Many of our most useful crops 
such as cotton, wheat, oats, tobacco, and potatoes are now known to be amphi- 
ploids. In the case of wheat, cotton, and tobacco, good evidence as to the actual 
parental species is available. The cultivated tobacco, Nicotiana digluta, was the 
first species to be artificially resynthesized from its parent species, N. tabaccum 
and N. glutinosa. The first Linnaean species to be artificially recreated was 



158 • THE EVIDENCE FOR EVOLUTION 

Galeopsis tetrahit, which was derived from a hybrid between G. pubescens and 
G. speciosa. Spartina townsendii and two amphiploids in the goats beard 
(Tragopogon), the latter two known to have arisen in the last 30 years, are 
examples of species that have originated in nature in recent times under human 
observation. Since the discovery of the colchicine technique for doubling 
chromosome numbers, a number of experimental amphiploids have been formed 
that must, by all the criteria commonly used, be regarded as new species. Some 
forty years ago, Bateson objected that despite all the discussion about the origin 
of species, no one had yet observed this event. Although the origin of species by 
polyploidy may be a special case, rather than what Bateson had in mind, the 
fact remains that man has now observed the origin of species in nature and has 
also synthesized his own new species. 

Domesticated Species 

Darwin opened his book The Origin of Species with a chapter on 
"Variation under domestication" and later summarized his studies in this area in 
the book entitled The Variation of Animals and Plants under Domestication. 
Domesticated species are still of considerable interest, for they give us a magni- 
fied although somewhat distorted view of evolution. Darwin's work, though 
significant even today, was marred by the lack of knowledge of the causes of 
variations and of their mode of inheritance. He recognized, however, the rele- 
vance of this type of study to the problem of the origin of species. A more 
sophisticated discussion couched in modern genetic terms is now possible, but 
the conclusions relating to the significance of domesticated species as evidence 
for evolution are little different. 

In brief, these conclusions are that domestic animals and plants are 
descended from wild species. In most cases they appear to have been derived 
from a single species, but some may have originated from species crosses. The 
numerous breeds or varieties have arisen as the result of both conscious and un- 
conscious artificial selection by man, and also, it must be added, by natural 
selection operating in the new environments provided by man. The origins of 
many domesticated species are obscured in the mists of antiquity or of prehistoric 
times. The dog, the horse, the pig, wheat, rice, and corn — these and many others 
were domesticated during times for which no historical records are available. In 
other cases, domestication is so recent that virtually a complete history of the 
process can be given. For example, fox and mink breeding are less than a century 
old yet already a number of varieties have been developed, and the fruit fly, 
Drosophila melanogaster, from which so much of our knowledge of heredity has 
been gained, also must be included in any list of recently domesticated species. 
Furthermore, new breeds or varieties of the older domesticated species continue 
to be created, such as the Santa Gertrudis cattle, the Minnesota No. 1, No. 2, and 
No. 3 hogs, and Thatcher wheat. 



GENETIC EVIDENCE • 159 

The breeds of dogs range from Chihuahuas and Schnauzers to Great 
Danes and St. Bernards; of horses, from Shetland ponies to Percherons — yet 
despite their great differences in size and other traits, all dogs are regarded as 
belonging to one species, as are all horses. The dogs belong to a single species 
because all the many breeds are capable of hybridizing except where extreme size 
differences intervene, and even then indirect exchange of genes takes place 
through intermediate breeds. Since the differences between some of the breeds 
of domestic animals appear to be much greater than those between some well- 
defined and reproductively isolated wild species, it may be wondered why these 
breeds have not become reproductively isolated also. Although no definitive 
answer can be given, a guess may be hazarded that even the oldest breeds have 
been established but a very short time, a matter of a few thousand years at most, 
and that this period has not been long enough for the numerous genetic differ- 
ences leading to reproductive isolation to have accumulated in the separate 
breeds. In other words, the differences, great as they appear to be, may still be 
controlled by comparatively few of the many genes in the species. 

The significance of domesticated species as evidence for evolution lies 
in the fact that they show that species have changed and can be changed. The 
numerous breeds exemplify on a small scale divergence or descent with modi- 
fication — in other words, evolution. 

Gene and Chromosome Homology 

Another type of genetic evidence for the relationship between species 
is drawn from a comparison of their chromosomes. In every individual, a set of 
maternal chromosomes is matched by a corresponding set from the father, and 
pairing or synapsis only occurs between the similar or homologous chromosomes 
of each set. Furthermore, these maternal and paternal chromosomes pair only in 
a very specific "gene by gene" fashion. Hence, if pairing occurs between the 
maternal and paternal chromosomes of a hybrid from a species cross, it is a rea- 
sonable assumption that the paired regions are homologous, containing similar 
genetic material. The best studies of this type have been conducted with species 
with giant salivary gland chromosomes belonging to the order Diptera and in- 
cluding fruit flies (Drosophila) , midges (Chironomus), mosquitos {Anopheles), 
and gnats (Sczara). The large size and banded structure of the salivary gland 
chromosomes permit the specific identification of given regions. Since somatic 
pairing occurs, the band by band pairing of homologous regions can be seen in 
great detail. In hybrids from the cross between Drosophila melano gaster and D. 
simulans, two morphologically similar species, most regions of the chromosomes 
can be seen to be alike and to be paired. Only a few regions show differences in. 
the banding structure, and these remain unpaired. Furthermore, genetic studies 
have shown that there are similarities in genetic behavior in the synapsed regions 
whereas the unpaired regions differ in their genetic contents. In general, species 



160 • THE EVIDENCE FOR EVOLUTION 

less alike morphologically than these two produce hybrids that have fewer 
homologous paired regions. The most obvious interpretation of these facts is that 
during the course of evolutionary divergence, the chromosomes, as well as the 
gross morphology, have been restructured and repatterned. Moreover, because of 
the specificity of chromosome pairing, chromosomal homologies are even more 
sensitive and reliable than anatomical homologies. 

Above, in passing, we mentioned the similarities in genetic behavior 
between homologous chromosome regions. This material constitutes still another 
link in the chain of evidence for evolution. In brief, it has been possible to show 
that similar mutant types in different species represent mutations of homologous 
genes. In some cases, these gene homologies have been established by crossing 
mutant types of each species and obtaining mutant hybrid offspring in the first 
generation. This result would not be obtained with nonhomologous recessive 
mutants (that is, mutants expressed only when present in double dose), for the 
hybrids would then be normal or wild type in appearance. In other cases, where 
hybridization is impossible, the evidence of necessity is less direct. However, the 
demonstration of the homology of individual genes in different species represents 
one of the most precise bits of evidence for their common ancestry yet available. 

The Hereditary Material 

The study of the chemical nature of the chromosomes from species 
ranging from viruses and bacteria to higher plants and animals has shown that 
they are composed of nucleoprotein, a combination of protein and nucleic acid. 
Nucleic acids are of two kinds: DNA or deoxyribonucleic acid and RNA or 
ribonucleic acid. DNA is found in the nucleus of cells while RNA may be found 
in both nucleus and cytoplasm. Chemically very similar, both have a backbone of 
a long chain of alternate sugar and phosphate molecules with purine and pyrimi- 
dine bases attached to the sugars as side groups. The differences lie in the sugars, 
deoxyribose in DNA and ribose in RNA, and in one of the four bases. Both 
have the purines, adenine and guanine, and the pyrimidine, cytosine, in common, 
but in DNA the other pyrimidine base is thymine; in RNA it is uracil. All of 
the available evidence indicates that the nucleic acids carry the hereditary blue- 
print from one generation to the next. In all but a few cases (for example, some 
plant viruses) DNA is the hereditary material while the RNA ordinarily seems 
to mediate protein synthesis. 

One type of evidence for the hereditary role of DNA comes from the 
discovery that the "transforming principle," which can produce inherited changes 
when added to bacterial cells, is DNA. Hereditary changes in the type of poly- 
saccharide capsule in pneumococci, for example, are induced by DNA from a 
related strain rather than by its polysaccharide. Furthermore, when a bacterial 



GENETIC EVIDENCE • 161 

cell is infected by a bacterial virus, the DNA from the virus penetrates the 
bacterium and initiates virus reproduction there, but the protein coat of the virus 
is left outside of the cell. 

DNA has been shown to be composed of two long strands coiled around 
each other to form a double helix (Fig. 15-2). The bases of one strand pair very 
precisely with the bases on the other. In fact, adenine pairs only with thymine, 
and guanine only with cytosine. Hence, the sequence of bases on one strand 
determines the sequence on the other, a fact that appears related to their power 
of self -duplication. It might seem that DNA, limited to just four bases, a single, 
simple type of sugar, and phosphate groups, would lack the complexity necessary 
to control the great variety of hereditary traits in hundreds of thousands of 




Fig. 15-2. Watson-Crick double helix model of the DNA molecule. S = sugar 
(deoxyribose). P = phosphate. Purine bases: A = adenine and G = guanine. 
Pyrimidine bases: T = thymine and C = cytosine. A always pairs with T, and 
GwithC. 



species. However, the order of the bases in the DNA molecule is not regular or 
repetitive, and the specificity and function of the genes appear to be determined 
by the sequence of the bases along the DNA chain. In this way an enormous 
variety of specifications can be encoded or spelled out. The picture now emerging 
is that DNA specificity is conferred on RNA, which moves into the cytoplasm 
where it controls protein synthesis. Thus, the DNA code is eventually imprinted 
on the enzymes, the protein compounds that carry on the bulk of the metabolic 
activities of the cell. 

The simple fact that the ultimate genetic material in nearly all species 
can be represented as variations on a theme in a single type of compound, DNA, 
makes evolution in all its ramifications more readily comprehensible. This fact 
points up the fundamental similarity among all living things, and the problem 
eventually will be to discover how DNA patterns have changed in the course of 
time to give rise to the great diversity of living species. 



162 • THE EVIDENCE FOR EVOLUTION 

SUMMARY < 



The discovery of genetic principles has led not only to 
an understanding of the mechanism of evolution but also to 
further evidence for evolution. Hybridization between distinct 
species has been repeatedly observed in plants and animals, and 
in the case of polyploids has led to the formation of new species. 
The creation of new species is, in itself, an insurmountable argu- 
ment against a static-species concept. The development of new 
breeds and varieties under domestication is still further evidence 
that species under selection pressure can and do change. The study 
of the genetic material itself has revealed homologies between dif- 
ferent species at all levels of organization, from chromosomal re- 
arrangements to DNA structure. Since DNA is the stuff of 
heredity, the basic question in the study of evolution is to deter- 
mine how in the course of time DNA patterns have changed. 

SUGGESTED READING 

Darwin, C, 1868. The variation of animals and plants under domestication. London. 

Davidson, J. N., 1957. The biochemistry of the nucleic acids, 3d ed. New York: 
Wiley. 

McElroy, W. D., and B. Glass, eds., 1957. The chemical basis of heredity. Balti- 
more: Johns Hopkins Press. 

Miintzing, A., 1959. "Darwin's views on variation under domestication in the light 
of present-day knowledge," Proc. Amer. Philosophical Society, 103:190- 
220. 

Stebbins, G. L., 1959. "The role of hybridization in evolution," Proc. Amer. Philo- 
sophical Society, 103:231-251. 

White, M. J. D., 1954. Animal cytology and evolution, 2d ed. New York: Cam- 
bridge University Press. 





PART 



m 



The Mechanism 

of Evolution 



The remainder of the book, which is devoted to the 
mechanism of evolution, may be regarded as a more extensive 
genetic argument for evolution even though it has not been writ- 
ten from that point of view. Before the mechanism of evolution 
is considered in detail, it may be helpful to state, rather briefly 
and without too many qualifications, the essential points in the 
current concept of evolution. The theoretical basis of modern evo- 
lutionary theory was developed primarily by R. A. Fisher, }. B. S. 
Haldane, and S. Wright. 



163 



164 • THE MECHANISM OF EVOLUTION 

Darwin believed that a cross between two unlike individuals re- 
sulted in a blending of their heredity and hence in a loss of variability. 
Mendel, however, demonstrated that heredity is particulate in nature rather 
than blending. Mendel's results led to the realization in 1908 by Hardy 
and Weinberg that random mating in a population where all types are 
equally favored does not result in a loss of variability, but that the variability 
remains constant from one generation to the next. This concept has come to be 
known as the Hardy-Weinberg law. 

If evolutionary change is to occur, new kinds of hereditary variation 
must appear. These changes in the hereditary material, known as mutations, 
have been shown to occur spontaneously at a very low frequency, which can be 
raised by various forms of radiant energy and by some chemical substances. 
Mutations are essentially random within the existing genetic system, and form 
the raw material of evolution. The knowledge of mutations, both genie and 
chromosomal, and of the mutation process is considerably greater today than it 
was a few decades ago. 

Natural selection determines the fate of new mutations and of the new 
gene combinations resulting from Mendelian recombination. Only the adaptively 
favorable genes or combinations of genes will persist and become incorporated 
into the breeding population. 

Evolution is a phenomenon occurring in populations, not in individuals. 
The evolving unit is a breeding population. If the size of the population is small, 
random loss or fixation of genes may occur, quite apart from the operation of 
natural selection. As a result of this "genetic drift," and also because of the 
greater likelihood of inbreeding, small populations are apt to be more homo- 
zygous than large, and consequently less able to adapt to changing environmental 
conditions. 

A species may consist of one large randomly mating population or, 
more often, of a number of more or less isolated breeding populations. A single 
large population remains quite variable and evolves as a unit. If each of a 
number of breeding populations is completely isolated from the others, evolution 
will proceed independently in each, the resultant of the pressures of mutation 
and selection and of the random effects of genetic drift. Between the extremes of 
complete isolation on the one hand and random mating on the other, all degrees 
of partial isolation are possible. Each population will then serve as an evolu- 
tionary experiment, which, if successful, may spread its influence to other popu- 
lations through the gene flow made possible by migration. If gene flow is too 
restricted, the more successful population may supplant others as the result of 
intergroup selection. Thus, the course of evolution may be influenced by the 
structure of the species population, the way in which it is subdivided into breed- 
ing populations, and the degree of isolation and gene flow among them. 



THE MECHANISM OF EVOLUTION • 165 

The great achievement of the population geneticists is that they have 
incorporated the four major factors causing gene frequency changes in popula- 
tions (mutation, selection, genetic drift, and migration) into a mathematical 
model that permits the consideration of the simultaneous effects of all of these 
factors. Even though these factors are as biologically diverse as mutation, via- 
bility, mating preferences, isolation, differential fertility and fecundity, and 
migration, they have all been evaluated in terms of their effects on gene fre- 
quencies. Evolution, therefore, is now considered to be essentially a series of 
changes in the kinds or frequencies of genes in populations, or more briefly, a 
shift in the Hardy- Weinberg equilibrium. Since this is the case, it is essential, if 
we are to understand the mechanism of evolution, that we gain some grasp of 
the genetics of populations. But first, we must understand the basic principles of 
genetics. 



CHAPTER 



16 

Mendel's Laws 



Thus far, we have considered the nature of the biological 
world and the theory that explains how it has achieved its present 
state — namely, the theory of evolution. The nature of the evi- 
dence in support of the theory of evolution has been reviewed, 
and some idea of the evolutionary changes that have occurred has 
been presented. The clearer it has become that evolution is a fact, 
the more urgent has become the need to explain how one species 
can evolve into another, and what forces operate to make evolu- 
tionary change possible. 

Darwin's proposed mechanism for evolution was the 
theory of natural selection. A major weakness of his theory, which 
he clearly recognized, was his lack of knowledge about the in- 
heritance of variations. Darwin based his theory of natural selec- 
tion on the differential survival and transmission of hereditary 
variations. Though Darwin studied heredity and variation inten- 
sively, as others did before and after him, he failed to find the 
key to the problem. The advent of the science of genetics has 
supplied some of the missing knowledge, and in the process has 
broadened and strengthened the theory of natural selection. 

The first steps toward an understanding of heredity were 
made by an obscure monk, Gregor Mendel, who experimented 
with the common garden pea in a small monastery garden. Alone, 
without a research team or even a grant for a research project, he 
worked out with beautiful simplicity and in detail the funda- 
mental laws governing the transmission of characters from parent 
to offspring in sexually reproducing plants and animals. A prob- 

166 



mendel's laws • 167 

lem that had intrigued and puzzled men for centuries was solved by a 
man who had twice failed his examinations to gain a teaching certificate. 
Yet his discoveries were apparently neither understood nor appreciated by 
the recognized scientists of the day, and their significance was not realized 
until 1900, some 35 years after the work had been completed and 
published. The study of heredity, or genetics, as it came to be called, is thus a 
science that, perhaps more than any other, belongs to the twentieth century. 
During its brief career, it has not only contributed to our basic understanding 
of the mechanism of heredity, with ramifications in every area of biological 
thought; it has transformed the face of the earth and added incalculable riches 
to the resources of the world through the widespread use of new and improved 
varieties of plants and animals developed through genetic research. 

The basic questions that Mendel answered were very simple. If a father 
and mother and their child are seen together, the resemblances of the youngster 
to his parents can be readily observed. But all children do not show the same 
degree of resemblance to each parent. Some appear to be the "spitting images" 
of their fathers; others, of their mothers. Most show some of the traits of both 
while some seem to show little resemblance to either parent. This strange and 
varying assortment of similarities and differences between parents and offspring 
had been the stumbling block to all who had previously attempted to study 
heredity. Any adequate theory of heredity must not only explain how father 
passes on his big brown eyes to junior, and mother contributes her widow's peak, 
but also where in the world he got that flaming red hair, the like of which has 
"never" been seen in either family. Genetics, then, is the study of the way in 
which these resemblances are passed from one generation to the next and of the 
mode of origin of the variations. 

Careful examination and observation of any group of organisms will 
show that each individual within the group is unique and clearly different from 
all the rest. Hence, any attempt to study heredity in a group is almost hope- 
lessly complex if an effort is made to study simultaneously all of the distinguish- 
ing characters of each individual. It is like trying to pitch a tent in a tornado — 
impossible to keep track of everything at once. Mendel's success, in large part, 
was due to the fact that, rather than trying to follow the great multiplicity of 
characters, he sought to answer the question of how a single trait with two well- 
defined alternative conditions, such as yellow or green peas, was transmitted 
from generation to generation. In this way, he reduced the problem to its 
simplest terms. Although knowledge of the physical basis of heredity was virtu- 
ally nonexistent at the time, Mendel realized that yellow or green seeds were 
not transmitted as such from one generation to the next, but that somewhere 
within the pollen and the ovule there were factors that controlled the tendency 
to develop one color or the other. Over the narrow physical bridge of pollen and 
ovule in plants, sperm and egg in animals, must pass all of the factors that 






168 • THE MECHANISM OF EVOLUTION 

determine not only the color of the seeds but also that a pea plant will never 
become a rose bush; not only the color of junior's hair and eyes but also that 
he develops into a man and not a mouse. 

Since every individual is the product of a developmental sequence con- 
trolled and influenced by both heredity and environment, the observed variations 
may be primarily due to heredity, or environment, or both. The old nature- 
nurture or heredity vs. environment controversy is virtually meaningless. With- 
out heredity, there is no organism at all, and it therefore must play a role in all 
that an organism is and does. Yet every organism develops in an environment of 
some sort, which is always present and whose role must always be considered in 
any assessment of the individual organism. However, all traits are not equally 
influenced by heredity and environment, for some are more subject to environ- 
mental modification than others. 

As Darwin pointed out, only the hereditary variations are important to 
evolution. We shall therefore not be concerned here with environmental varia- 
tion, although from the experimental and practical standpoint it is always a 
factor to be reckoned with. Our problem is to account for the inheritance of 
both similarities and differences. Actual traits, of course, are not inherited as 
such. Your eyes are the result of a period of embryological development from 
the fertilized egg, which has no eyes at all; therefore, they cannot be trans- 
mitted directly. We want to know what is transmitted and how it is transmitted 
from one generation to the next. 

Segregation 

Mendel studied, in all, seven traits in the garden pea, each with two 
well-defined alternative conditions. As in much biological research, a good deal 
of his success can be laid to his choice of a suitable experimental organism. The 
pea was extensively cultivated, and many varieties with different hereditary traits 
were readily available. The pea is normally self-fertilized, so that the danger of 
contamination by foreign pollen was negligible, yet it is fully fertile when 
crossed. Furthermore, he kept accurate records of the pedigrees of each of his 
plants, and classified and counted all of the progeny from his crosses. This arith- 
metic approach gave him more insight into the hereditary process than was pos- 
sible for those who merely classified without counting. Finally, as is also often 
the case in research, there was an element of luck involved. Although this is 
getting ahead of the story somewhat, there are only seven pairs of chromosomes 
in the pea, and each of the traits Mendel chose happened to be controlled by a 
different pair. If any two traits had been controlled by the same chromosome 
pair, the seemingly anomalous results he then would have obtained might have 
prevented him from breaking through to the generalizations known as Mendel's 
laws. The chance of such a choice of traits is, roughly, only 1 in 200. 



mendel's laws • 169 



What were the results Mendel obtained when he crossed two pure lines 
differing in a single trait? One of his crosses was made between a line that pro- 
duced only full, round peas and another that produced only wrinkled peas 
(Fig. 16-1). From this cross, all of the progeny, known as the first filial or F x 
generation, were like the round parent. For each of the other characters, Mendel 



{ 



Round y Wrinkled 





Round 



3 Round : 1 Wrinkled 




All wrinkled 



3 Round : 1 Wrinkled 



Fig. 16-1. Mendel's results with a monohybrid cross involv- 
ing round and wrinkled peas. 

found that the Fj progeny from crosses between pure lines were also all like one 
of the parents. He therefore called dominant those traits that were expressed in 
the F 1} and recessive those traits not appearing in the F x . 

The Fj progeny were then self-fertilized to produce the F 2 generation. 
In the F 2 , a ratio of 3 round plants to 1 wrinkled was obtained. The F 2 wrinkled 
plants all bred true for wrinkled, but of the F 2 round plants, one-third bred true 
while two-thirds behaved like the F l5 giving 3 round to 1 wrinkled offspring. 



170 • THE MECHANISM OF EVOLUTION 



From these results, Mendel drew certain inferences. Since wrinkled was 
present in one of the parents but was not observed at all in the F l5 some sort of 
a factor for it must have been present but not expressed in the F x generation. 
Therefore, the ¥ r carried a factor for wrinkled as well as for round, and hence 
was a hybrid. Since the wrinkled trait appeared unchanged in the F 2 , passage of 
the factor for wrinkled through the F 1 hybrid did not affect its nature or purity. 
See Fig. 16-2. 



{ 




RR 
Round 



& 



rr 
Wrinkled 



P 1 gametes 



{ 



\^/ 



F 1 gametes j R : 2 r 



F, o* 




Rr 
Round 



V gametes 

1 X R 


r 


R 


m m rr 


$ 




Round 


Round 


r 


• * 


^ " 




Round 


Wrinkled 



F2 breeding 
behavior RR 



2Rr 



{ 



RR 



F 3l Round 



3R : Irr 
3 Round : 1 Wrinkled 



Wrinkled 



Fig. 16-2. Mendel's interpretation ot the results trom the mono- 
hybrid cross with round and wrinkled peas. 



MENDEL'S LAWS • 171 



Furthermore, the reappearance of the pure-breeding wrinkled and pure- 
breeding round plants in the F 2 meant that the factors for round and wrinkled, 
which were present together in the ¥ t hybrids, must have been separated or 
segregated before the formation of the F 2 . Therefore, although the ¥ x plants 
were hybrids, their gametes, or sex cells, must have been pure. The gametes must 
carry either the dominant round factor or the recessive wrinkled factor, and must 
be of two. kinds. The 3 : 1 ratio could easily be explained if the two kinds of 
gametes were produced in equal numbers and union of the gametes at fertiliza- 
tion occurred at random. These results and conclusions led to the formulation of 
what is now known as Mendel's first law, the prijidpIeMf segregation. It can be 
stated as follows: When a hybrid reproduces, it transmits with equal frequency 
either the dominant character of one parent or the recessive character of the 
other, but not both. 

These concepts can be more readily visualized and handled if they are 
written out in a convenient short form. 

Let R = factor for round 
r = factor for wrinkled 
Then a pure plant for round would be RR, and for wrinkled, rr. A cross between 
the two, known as a monohybrid cross, can be outlined as follows, where P x is 
the first parental generation: 

Pi RR (round) X rr (wrinkled) 

\ S 

Pi gametes all R all r 

Fi all Rr (round) 

s\ 

Fi gametes 1/2 R : 1/2 r 



\ Fief 
\ gam 
Fi 9 \ 
gam \ 


R 


r 


R 


RR 

round 


Rr 
round 


r 


rR 
round 


rr 
wrinkled 



From the checkerboard used to get the F 2 , it is readily seen why a 3 : 1 F 2 ratio is 
obtained, and also why 2 of the 3 round individuals must be hybrids. 

At this point it may be well to introduce a few more terms and con- 
cepts. A true-breeding organism, such as an RR round pea plant or an rr 
wrinkled plant, is said to be homozygous; a hybrid plant, such as an Rr plant, 
which produces two kinds of gametes, is said to be heterozygous. The term 
"factor" used by Mendel has been to a large extent supplanted by the word 
tfgene." Learning genetics is much like learning a new language, and just to 
show how the jargon is used, the cross outlined above is said to be between a 



172 • THE MECHANISM OF EVOLUTION 

line that is homozygous for the gene for round and one homozygous for 
wrinkled to give a heterozygous round ¥ 1 . When inbred, the ¥ x produces an F 2 
consisting of 1 homozygous wrinkled and 3 round, of which % are homozygous 
and % heterozygous. 

The concepts of genotype and phenotype are related to each other and 
are fundamental. The sum total of all the traits expressed by the individual — 
morphological, physiological, psychological, biochemical, etc. — is said to com- 
prise his phenotype. The sum total of all of the genes an individual carries, 
received from his parents and transmissable to his offspring, is said to be his 
genotype. The phenotype is the product of the genes in the genotype acting 
within a particular environment. The same genotype placed in different environ- 
ments — for example, cuttings from a single plant reared under different climatic 
conditions — will give different phenotypes. Yet the same phenotype may be 
produced by different genotypes, as for example the RR and Rr round peas. 

Dominance is not a universal phenomenon. The Rr peas, for example, 
are as round as the RR seeds, but microscopic examination of the starch grains 
shows them to be intermediate in form between those from RR and" rr seeds. 
Also, a cross between a red variety and a white variety of zinnias gives a pink 
Fj hybrid, and an F 2 of 1 red, 2 pink, and 1 white. Such examples can be multi- 
plied many times to show that all degrees of dominance exist; it may be com- 
plete, partial, or lacking. 

A human trait inherited in accordance with the simple rules outlined 
above is albinism. Albinos in man are characterized by a deficiency in pigmenta- 
tion and, frequently, eye defects, in addition to other anomalies. Albinism is 
due to a recessive gene in the homozygous condition. Although it is a rare condi- 
tion, there are many normally pigmented people who carry this gene in the 
heterozygous condition. A simple method for determining what proportion are 
carriers is to discover what proportion of the marriages of albinos to unrelated 
normally pigmented people result in the production of albino children. Such 
matings are known as "test crosses," since crosses to the homozygous recessive 
quickly reveal the genotype of the normally pigmented parent. If the normal 
parent is homozygous, all of the children will be pigmented. 

Pi CC x cc 

normally albino 

pigmented j 

Pj gametes all C all c 



Y x all Cc 

normally 
pigmented 
but carriers 




mendel's laws • 173 

If the normal parent is a heterozygous carrier of the albino gene, however, half 
of the children, on the average, will be albino. 

Pi Cc 

normally 
pigmented 
* 
P 1 gametes i/ 2 C' Vl c 



l/ 2 Cc <<. \l/ 2 cc 

normally albinos 

pigmented carriers 

Such studies have shown that although only about 1 European in 
20,000 is an albino, approximately 1 in 70 is a heterozygous carrier of the gene 
for albinism. Thus, the test cross, or the back cross to the recessive, as it is also 
called, is the most direct method of ascertaining the genotype of an individual 
whose genotype is unknown. 

Independent Assortment 

After Mendel had established the way in which single traits were trans- 
mitted from generation to generation, his next question became: What happens 
if individuals differing in two traits are crossed ? In one such cross, for example, 
one of the parents had wrinkled and yellow seeds while the other bred true for 
round, green seeds. This cross produced a uniform F l5 all having round and 
yellow seeds, these being the dominants. In the F 2 , however, four phenotypes 
appeared, two like the original parents plus the other two possible combinations, 
round yellow and green wrinkled. Furthermore, they occurred in a definite ratio 
of 9 : 3 : 3 : i . Mendel inferred from these results that the segregations of the 
factors governing these two traits were independent of each other. The 3:1 
segregation of one factor pair (green-yellow) was completely independent of the 
3:1 segregation of the other factor pair (round-wrinkled). The 9:3:3:1 ratio 
then occurs because, of the % of the seeds which are round, % are yellow and 
Vi green; of the J4 which are wrinkled, % will also be yellow and l/i green. 
Hence, 

% X % = % 6 round yellow 

3/ 4 X y 4 = % 6 round green 

Vi X Va — %6 wrinkled yellow 

Y4 X 1/4 = Y 16 wrinkled green 

These results formed the basis of Mendel's second law, the principle of inde- 
pendent assortment. The law, stated briefly, is that_the_segregation of one factor 
pair occurs independently of any other factor pair. 



174 • THE MECHANISM OF EVOLUTION 




3-0 




YYrr 
Yellow wrinkled 



yyRR 
Green round 



P 1 gametes 



1 



Yr yR 

x o" 

YyRr 
Yellow round 



IVD lv. In 1 



F, gametes JYR ■. \Yr : jyR ■. \yr 



gametes 

F, ? 

gametes 



r,< 





YR 


Yr 


yR 


y 


YR 


O 

YYRR 


o 

YYRr 


o 

YyRR 


O 

YyRr 


Yr 


o 

YYRr 


YYrr 


o 

YyRr 


Yyrr 


yR 


o 

YyRR 


o 

YyRr 


• 

yyRR 


• 

yyR/ 


yr 


o 

YyRr 


Yyrr 


• 




yyrr 



9 Yellow round : 3 Yellow wrinkled : 3 Green round : 1 Green wri 
Fig. 16-3. A dihybrid cross in peas. 



nkled 



mendel's laws • 175 

This dihybrid cross, as it is called, can be outlined as follows: 
Pi YYrr 



V x gametes 



YYrr 
yellow wrinkled 

I 

Yr 



yyRR 

green round 

4- 

yR 



F x YyRr 

all yellow round 

Fi gametes l/ 4 YR: l/ 4 Yr: l/ 4 yR: l/ 4 yr 

Since the segregations are independent of each other, all possible combinations of 
the dominant and recessive genes are formed with equal frequency in the F a 
male and female gametes. See Fig. 16-3. 



\ F lC f 
\gam 
Fi 9\ 
gam \ 


YR 


Yr 


yR 


yr 


YR 


YYRR 


YYRr 


YyRR 


YyRr 


Yr 


YYRr 


YYrr 


YyRr 


Yyrr 


yR 


YyRR 


YyRr 


yyRR 


yyRr 


yr 


YyRr 


Yyrr 


yyRr 


yyrr 



The checkerboard should be examined carefully. The origin of the four 
phenotypes and their ratio will then be obvious: % 6 of the individuals have at 
least one dominant Y and one dominant R; % 6 are homozygous yy but carry 
dominant R; % 6 are rr but carry dominant Y; and only y 1Q of the plants are 
homozygous for both recessives. Furthermore, though there are only four pheno- 
types, they result from nine distinct and different genotypes, four of which will 
breed true. It should be noted that the same results in the F 2 would have been 
obtained if the original cross had been 

Pi YYRR X yyrr 

yellow round green wrinkled 



► SUMMARY 



Variation is the working material of evolution, but not 
all variations are inherited; only the hereditary variations are of 
significance in evolution. Therefore, the distinction between 
phenotype and genotype is fundamental. Every individual carries 



176 • THE MECHANISM OF EVOLUTION 

two complete sets of genes, one set coming from the mother, the 
other from the father. Each gamete carries only one complete set 
of genes. Mendel discovered the orderly way in which these genes 
are transmitted from one generation to the next. Each pair of 
factors or genes segregates prior to gamete formation and then 
combines at random while the different pairs of genes segregate 
and recombine independently of one another. These principles 
form the genetic basis of variation through the recombination of 
genes. Even though the expression of some genes may at times be 
masked due to dominance, they are not lost but may reappear in 
subsequent generations. Through genetic recombination an almost 
infinite number of new genotypes can be formed on which natural 
selection can act. 
"o 
SUGGESTED READING 

The birth of genetics. Mendel-de Vries-Correns-Tschermak. Supplement to Genetics 

35(5), Part 2. 
Colin, E. C, 1956. Elements of genetics, 3d ed. New York: McGraw-Hill. 
Sinnott, E. W., L. C. Dunn, and Th. Dobzhansky, 1958. Principles of genetics, 5th 

ed. New York: McGraw-Hill. Appendix contains English translation of 

Mendel's original paper. 
Snyder, L. H. and P. R. David, 1957. The principles of heredity, 5th ed. Boston: 

Heath. 
Srb, A. and R. D.Owen, 1952. General genetics. San Francisco: Freeman. 
Stern, C, I960. Principles of human genetics, 2d ed. San Francisco: Freeman. 
Waddington, C. H., 1939. An introduction to modern genetics. London: Allen and 

Unwin. 



CHAPTER 



17 



Variation Due to 

Recombination 



Multiple Alleles 

Mendelian inheritance is particulate, the particulate genes 
retaining their identity in crosses. Segregation and recombination 
form the basis of much of the variability in a species population. 
Thus far, we have considered alternative forms, or alleles, of the 
same gene to t>e of just two kinds, exemplified by the dominant 
yellow (F) and its recessive allele (j), or the dominant round 
(R) and its recessive allele wrinkled (r). Numerous studies have 
shown, however, that a given gene can exist in a number of dif- 
ferent alternative conditions; hence a whole set of alleles may 
exist rather than only a dominant and a recessive. In some cases, 
there may be as many as forty of these multiple alleles, as they are 
called, in a single set — that is, forty different forms of the same 
gene, each with its own distinguishable phenotypic effects. How- 
ever, any diploid individual can carry in the cells of his body only 
two of these alleles at the most, while each of his gametes can 
carry but one. 

Multiple alleles open up new ranges in the possibilities 
for genetic recombination. In the ABO blood groups in man, for 
example, three major alleles determine the blood types, the genes 
being I A , l B , and 1°. The blood types and the genie combinations 
producing them are as follows: 



177 



178 • THE MECHANISM OF EVOLUTION 

blood type genotype 
(phenotype) 

O 1° 1° 

A I A I A or I A I° 

B P1 B or I B I° 

AB I A I B 

One added allele increases the number of possible genotypes from 3 to 
6, and increases the phenotypes to 4 from the two seen in the F 2 of a mono- 
hybrid cross with dominance. Note that I A and l B are both dominant to 1°, but 
not to each other. 

Another example may be taken from the C gene in the rabbit. Four 
alleles at this locus are the following: 

C — full color 
c ch - Chinchilla 
c h = Himalayan 
c = albino 

The C gene produces the familiar coat of the wild rabbit; c ch , a pearly gray ani- 
mal; c h , a white rabbit with black extremities; and c, a pure white rabbit with 
pink eyes. See Fig. 17-1. 

With four alleles, 10 distinct genotypes but only four color phenotypes 
are possible, since the dominance relations show C > c ch > c h > c. The number of 

different genotypes possible with n alleles can be shown to equal . Hence, 

the variability due to multiple alleles is by no means trivial and increases very 
rapidly as the number of alleles increases. 

number of number of 

alleles (n) possible genotypes 

1 1 

2 3 

3 6 

4 10 

5 15 

6 21 
10 55 
20 210 
40 820 

These possibilities are restricted to just one kind of gene. When it is remem- 
bered that the total number of genes in the genotype must be in the thousands, 
and that each gene may have several forms, then the number of combinations 



VARIATION DUE TO RECOMBINATION • 179 

possible among these different sets of multiple alleles becomes simply enormous 
— far greater than the number of individuals in the species. The wonder, per- 
haps, is not that two individuals in a species never look exactly alike, but that 
they resemble each other as much as they do. 

One further aspect of multiple allelism warrants mention. Each of the 
four C genes has a distinctly different effect on the phenotype. Yet in specially 
studied cases, it has been demonstrated that genes of different origin producing 
the same gross phenotypes, which cannot be distinguished from* one another by 




Fig. 17-1. Variation in rabbits due to multiple alleles. Top: left, full color; 
right, chinchilla. Bottom: left, Himalayan; right, albino. (Courtesy of Snyder 

and David.) 



inspection, nevertheless have subtly different effects, either physiologically or in 
their interaction with other genes in the genotype, and hence must be regarded 
as alleles rather than one and the same gene. These genes with equivalent gross 
phenotypic effects that are nonetheless demonstrably different are known as iso- 
alleles. For example, in the fruit fly, a mutant type with an interrupted wing 
vein, known as cubitus interruptus fa), has been crossed to various flies of dif- 
ferent origin, all with normal wing venation, and hence carrying wild-type alleles 
of the ci gene. However, since the expression of these wild-type genes in heter- 
ozygous combination with ci showed different degrees of effect on the cubitus 
vein, these wild-type genes are therefore isoalleles, and were designated as + l5 
+ 2 , and + 3 . Because of the difficulties of detection, the amount of isoallelism is 
not easily determined, but it is probably quite common, and contributes to the 
available variability in a more subtle way. 



180 • THE MECHANISM OF EVOLUTION 

Background Effects 

Thus far we have considered the gene to act independently in producing 
a trait, with a one-to-one relation between gene and character. Actually, any trait 
is produced by the action of many different genes plus the effects of the environ- 
ment. Hence, not only the numbers of combinations of genes, but the possi- 
bilities for interaction between them and between the genes and the environment 
must also be considered, for genes do not act in a vacuum. In the snapdragon an 
ivory variety (rr) and a red variety (RR) are known. The F x hybrid (Rr), if 
grown in bright light at a low temperature, is red; if grown in the shade at a 
high temperature, it is ivory. Thus the same genotype in different environments 
gives different phenotypes, and the dominance relations can only be defined by 
specifying the environmental conditions. Brachyury, a short-tail mutation in the 
mouse, behaves as a dominant in the European house mouse, Mus musculus, but 
as a recessive in the Asiatic house mouse, Mus bactrianus, when the same mutant 
male is crossed to females of both species. In this case the same gene placed on 
different genetic backgrounds rather than in different environments produces 
different phenotypes. 

Recombination and Interaction 

To illustrate the point that the combined action of many genes is re- 
sponsible for a single trait, let us consider the coat color in mink, Mustela vison. 
The rich, dark brown coat of the wild mink is the product of the genotype, PP 
Iplp AlAl BB BgBg BiBi CC 00 ss ff eb eb cm cm. These genes are known to 
affect coat color because mutant forms of each have been discovered; undoubt- 
edly still others will be identified when mutant forms of them are found. It is 
one of the peculiarities of Mendelian genetics that the individual gene can be 
identified only when two alternative forms of the gene exist. Thus, in a sense, 
the wild-type gene is an inference from the mutant allele. The mutant alleles of 
the genes listed above are as follows : 

genotype name genotype name 



PP 


— Platinum 


c» c H 


— Albino 


ip ip 


— Imperial platinum 


00 


— Goofus 


al al 


— Aleutian 


S 


— Black cross 


bb 


— Brown-eyed pastel 


F 


— Blue frost 


b R h 


— Green-eyed pastel 


Eb 


— Ebony 


bi bi 


— Imperial pastel 


Cm 


— Colmira 



Imagine, if you will, the possible color combinations that could be produced by^ 
suitable crosses. Some of these combinations have already been produced, with 
spectacular results, especially in the names they have received. 



VARIATION DUE TO RECOMBINATION -181 

Ffpp — Breath of spring platinum 
Ffbb — Breath of spring pastel 
al al ip ip — Sapphire 
bbpp — Platinum blond 

Although this particular type has not been synthesized, it would be most interest- 
ing to see an animal of genotype, bg bg oo, which should probably be called 
a green-eyed goofus. 

When different genes affect different traits, it is relatively simple to 
predict the outcome of crosses involving these genes. However, when different 
genes affect the same trait, prediction is more difficult because of the interactions 
between the genes. Even the simplest such cross, involving just two gene pairs, 
can illustrate the complexities. In chickens, for example, the following results 
have been obtained in comb shape (see Fig. 17-2) : 

P x rose X pea 

F x walnut 

1 
F 2 9 walnut : 3 rose : 3 pea : 1 single 

This cross is obviously of the dihybrid type because a 9:3:3:1 ratio is obtained. 
The relationships are shown below: 

phenotype genotype 
Walnut R- P- 

Rose R- pp 

Pea r r P— 

Single rr pp 

A somewhat more complex example of interaction can be drawn from 
the mouse: 

P x black X albino 

1 
Fi agouti (wild type) 

F 2 9 agouti : 3 black : 4 albino 

The Fj agouti appears to be a throw-back to the ancestral wild-type mouse. 
However, the black and albino reappear in the F 2 , which again suggests a two- 
factor or dihybrid cross, but with a somewhat aberrant 9:3:4 ratio. The explana- 
tion: 

phenotype genotype 

Agouti C- A- 

Black C- aa 

Albino cc A-, and cc aa 



182 • THE MECHANISM OF EVOLUTION 







Fig. 17-2. Variation in comb shape in fowl due to the interactions be- 
tween two pairs of alleles. A, rose. B, pea. C, walnut. D, single. 
(With permission of Srb and Owen.) 



The difference from the previous cross lies in the fact that individuals homo- 
zygous for cc have no pigment whatever, no matter what other genes for pig- 
ment production may be present. In this case, then, the recessive c gene masks 
the expression of both the A and the a genes. In a sense, this phenomenon is 
like dominance in that one type of gene suppresses another, but since it involves 
different gene pairs rather than alleles, it has been called epistasis. 

One last example may serve to illustrate still another ratio and give 
some insight into the mechanism of action of these genes. Certain varieties of 
white clover produce fairly high amounts of cyanide while others have a low 
cyanide content. A cross between two low-cyanide varieties gave the following 
results : 



VARIATION DUE TO RECOMBINATION • 183 

Pi low strain A X low strain B 

i 
F 1 high in cyanide 

F 2 9 high : 7 low 

The chemistry of cyanide production in clover is fairly well understood, 
and may be outlined as follows : 

gene L gene H 

4- 4- 

precursor enzyme L substrate enzyme H cyanide 

substance * (cyanogenic ' 

glucoside) 

Thus strains A and B are both low but for different reasons. Strain A with geno- 
type LLhh lacks enzyme H; B of genotype HHH cannot form enzyme L. The 
proof of these statements comes from testing the F 2 for cyanide in the manner 
shown below. 



proportion 


leaf extract 


leaf extract 


leaf extract 


genotyf 


ofF 2 


alone 


+ substrate 


+ enzyme H 




9 


+ 


+ 


+ 


L-H- 


3 





+ 





UH- 


3 








+ 


L-hh 


1 











llhh 



Here the nature of the interaction is quite clear. A chain of synthesis is 
involved that, if broken at any point, produces the same phenotype, low cyanide 
content. Each step in the chain depends on the preceding steps. If the phenotypes 
differed for each type of interruption — for example, if the substances accumu- 
lated at the blockage points differed in color — then further genotypes could be 
detected phenotypically. 

This material on recombination brings out one of the main advantages 
of sexual reproduction; namely, the formation of gametes with a random sample 
of one allele from each of the thousands of allelic pairs makes possible a vari- 
ability or plasticity that is impossible without sex. Individuals reproducing 
asexually leave descendants with the same genotype as their own, but with sexual 
reproduction, new gene combinations are always produced at fertilization. These 
new genotypes do not simply involve new ways of adding old traits together. 
Through the interactions of the genes in these combinations, very different new 
types of individuals may emerge, some of which may have real advantages over 
their parents. Hence in both natural evolution and controlled evolution or plant 
and animal breeding, segregation, independent assortment, recombination, and 
interaction of genes provide a potent means of progress toward better adapted or 
more useful plants and animals. 



184 • THE MECHANISM OF EVOLUTION 

SUMMARY < 



Genes at a given locus are not necessarily confined to 
just two alternatives, the dominant and recessive alleles, but may 
consist of a whole series of multiple alleles. Though each diploid 
individual will have, at most, only two alleles and each gamete 
only one, the possibilities for variability within a population are 
greatly extended by multiple alellism. The genetic variation of a 
population is further enhanced and diversified by the variety of 
interactions among genes at different loci. These epistatic inter- 
actions add still another dimension to the possibilities for genetic 
variation stemming from the recombination of genes. Since evolu- 
tionary change is dependent upon the available genetic variability, 
the variation arising from recombination plays a significant role 
in the evolution of sexually reproducing species. 

SUGGESTED READING 

Demerec, M., ed., 1958. "Exchange of genetic material: Mechanisms and conse- 
quences," Cold Spring Harbor Symp. Quant. Biol., Vol. 23. Long Island 
Biological Ass'n. 

See also references at the end of Chapter 16. 



CHAPTER 



18 



The Physical Basis 

of Evolution 



The hereditary mechanism elucidated by Mendel ac- 
counted for the transmission of similarities and the . origin of 
changes from one generation to the next. Since evolution involves 
change over successive generations, it obviously is related to the 
hereditary mechanism. In fact, the mechanism of heredity is the 
mechanism of evolution as well. Both heredity and evolution have 
the same physical basis, and it is time now that we consider the 
physical basis of evolution. The factors of Mendel were merely 
symbols or abstractions. He had no idea of where they were or of 
what they were, but postulated their existence in order to explain 
his data. 

In the interval between the publication of Mendel's re- 
sults and their rediscovery, the study of cells, or cytology, pro- 
gressed tremendously. The cell theory had been formulated only 
a few decades before Mendel's time, and the cells were then rec- 
ognized as the basic structural units in both animals and plants, 
but little was known of the details of their structure or function. 
The chromosomes in the nucleus were not even named until 1888, 
long after Mendel's work. That nuclei came from existing nuclei 
was only recognized about 1875 by Strasburger. The process by 
which new nuclei are formed was called mitosis. 

Mitosis 

Mitosis is a continuous process, which, for descriptive 
purposes, has been divided into phases or stages known as 



185 



186 • THE MECHANISM OF EVOLUTION 




INTERPHASE 
nucleus 




Early PROPHASE 
showing chromatids 




Late PROPHASE 

Chromosomes shorten, 

spindle forms 




METAPHASE 
Chromosomes line up 
at equatorial plate 



TELOPHASE 
Formation of 
2 daughter nuclei 



ANAPHASE 
Chromatids separate 



Fig. 18-1. Mitosis in nucleus with three pairs of chromosomes. 

prophase, metaphase, anaphase, and telophase. The interphase between successive 
mitoses has been called the resting stage, but a more suitable term perhaps is the 
metabolic stage. During mitosis each of the chromosomes in the nucleus under- 
goes a longitudinal doubling to form two chromatids (see Fig. 18-1). The 
chromatids of each chromosome separate during anaphase and move as chromo- 
somes to the opposite ends of the cell where they form two similar groups that 



THE PHYSICAL BASIS OF EVOLUTION • 187 




PROPHASE I 

Each chromosome splits 

into 2 chromatids and 

homologous chromosomes 

pair (synapsis) to form 

tetrads 




PROPHASE I 

Tetrads showing 

chiasmata 



ANAPHASE I 

Homologous chromosomes 

of each pair separate 




PROPHASE II 





c,— * vC 

ANAPHASE II 
Chromatids of each dyad separate 



Four haploid 

gamete 

nuclei 




Fig. 18-2. Meiosis in gametocyte with three pairs of chromosomes. 



^ 



188 • THE MECHANISM OF EVOLUTION 

then reconstitute two new daughter nuclei. These nuclei become the centers of 
two new cells when a new cell membrane forms between them. The chromosome 
material in the new cells is similar and is also like that of the original mother 
cell. Mitosis is thus a precise means of self-duplication of the chromosomes, and 
all of the cells in the body produced by this process should have the same 
chromosome content. 

Life Cycle in Animals 

Each of us was formed by the fertilization of an egg or ovum by a 
sperm cell. The egg carries a set of chromosomes from the mother; the sperm, a 
similar set from the father. The fertilized egg or zygote and all the cells derived 
by mitosis from it thus carry two sets of chromosomes. If no reduction in number 
occurred prior to the next fertilization, the number of sets of chromosomes 
would double in each generation. However, a reduction in number does occur 
during the process of meiosis (Fig. 18-2), which may be regarded as a modifica- 
tion of mitosis. Thus the gametes, sperm and egg, carry a single set of chromo- 
somes, one of each type, and are said to be In or haploid. The body or somatic 
cells with two sets or a pair of each type of chromosome are said to be 2n or 
diploid. 

In the formation of sperm and egg cells in animals, a process known as 
gametogenesis, nuclear behavior is basically similar in males and females but in 
other ways spermatogenesis and oogenesis differ. In the testis, stem cells known 
as spermatogonia divide mitotically. Some of these cells continue to function as 
stem cells, while othe'rs enlarge somewhat to form primary spermatocytes. The 
first meiotic division of a primary spermatocyte then gives rise to two secondary 
spermatocytes. With the second meiotic division, four spermatids are formed. 
Metamorphosis of the spermatids, during which much of the cytoplasm is lost 
and a flagellum or tail is formed, leads to the formation of four functional 
spermatozoa. 

The oogonia in the ovary are fewer in number than the spermatogonia. 
An oogonium, through the accumulation of cytoplasmic material, enlarges greatly 
to form a primary oocyte. The first meiotic division is equal with respect to the 
nuclei, but the great bulk of the cytoplasm goes to one cell, and the other nucleus 
with very little cytoplasm is pinched off as the first polar body. The second 
meiotic division is also unequal cytoplasmically, so that an egg and the second 
polar body result. Thus oogenesis gives rise to only one functional egg cell even 
though as in spermatogenesis four cells result from the meiotic divisions. In 
higher animals the haploid condition is confined to the gametes themselves. 
There is an alternation between haploid and diploid conditions each generation, 
but the diploid condition restored at fertilization prevails during virtually all of 
the life cycle. 



THE PHYSICAL BASIS OF EVOLUTION • 189 




fertilization 




Fig. 18-3. The life cycle of an angiosperm (corn). (With permission of Wilson 

and Loomis.) 



Life Cycle in Plants 

Among higher plants an alternation of generations also exists in the 
life cycle. Two distinct stages are found, a diploid sporophyte and a haploid 
gametophyte. The gametophyte in mosses and ferns is quite prominent, but in 
the flowering plants it consists of just a few cells, and the plant body is the 
sporophyte generation. 

The meiotic divisions occur during the formation of haploid spores by 
the sporophyte. The spores, by a series of mitotic divisions, produce the haploid 
male and female gametophytes, which in turn produce haploid gametes. Union 
of the gametes forms a zygote that then develops into the diploid sporophyte. 

In angiosperms (see Fig. 18-3), the sporophyte or plant bears two 
kinds of spores, usually within the same flower. The male spores or microspores 



190 • THE MECHANISM OF EVOLUTION 

are formed in the anthers of the flower; the female spores or megaspores develop 
in the ovules of the pistil of the flower. The stamens and pistil are surrounded 
by accessory flower parts, the petals and sepals. 

In the anther, microspore mother cells enlarge and undergo two meiotic 
divisions to form a tetrad of male spores. The haploid unicellular male spore 
then undergoes a mitotic division to form a tube nucleus and a generative 
nucleus. This binucleate structure, the pollen grain, is the male gametophyte. 

The female spores form from megaspore mother cells. Each ovule con- 
tains a megaspore mother cell that divides meiotically to form a row of four 
cells. Three of these cells degenerate, but the fourth enlarges to form a func- 
tional female spore. The haploid nucleus divides mitotically to form a two-, 
four-, and finally eight-nucleate embryo sac. Three nuclei collect at each end, and 
one of the cells at one end becomes the egg. The mature embryo sac at this stage 
is the female gametophyte, consisting of the egg nucleus plus two synergid 
nuclei at one end, two polar nuclei at the center, and three antipodals at the 
other end. 

The pollen grain, after landing on the end of the pistil, breaks open, 
and the pollen tube grows down through the tissues of the pistil toward the 
ovule. As the tube, containing both tube and generative nuclei, approaches the 
ovule, the generative nucleus divides by mitosis to form two sperm nuclei. When 
the pollen tube enters the embryo sac, the tube nucleus disintegrates and a 
double fertilization occurs. One sperm nucleus fertilizes the egg to form the 
diploid zygote; the other unites with the two polar nuclei at the center of the 
embryo sac to form the 3w or triploid endosperm, a tissue for food storage. The 
zygote then develops into the new diploid sporophyte generation. 

Meiosis 

Meiosis, in the simplest terms, consists of two nuclear divisions during 
which the chromosomes divide only once. Most of the unique features in meiosis 
occur during the prophase of the first division. During this time, the two mem- 
bers of each pair of chromosomes come to lie side by side. Since by the time of 
this synapsis each chromosome has duplicated into two halves or chromatids, a 
tetrad of four chromatids is formed. Exact reciprocal exchanges between two 
nonsister chromatids frequently occur. In this way a portion of a maternal 
chromatid is transferred to a paternal chromatid and vice versa. These exchanges 
are detected cytologically as chiasmata in late prophase. 

At anaphase the homologous chromosomes of each pair separate to 
form dyads of sister chromatids, except in regions where exchanges have oc- 
curred. In these regions both maternal and paternal segments are present. At the 
second anaphase the centromere holding sister chromatids together divides and 
the chromatids of each dyad go to opposite poles, no further duplication of the 



THE PHYSICAL BASIS OF EVOLUTION • 191 

chromosomes having occurred. Hence each chromatid in a tetrad comes to lie in 
a different nucleus. A quartet of cells is formed, each cell with one complete set 
of chromosomes rather than the two present in the original cell. 

Sex Determination 

The precision observed in the distribution of the chromosomes at mitosis 
and meiosis suggested to the German biologist Weismann toward the close of 
the nineteenth century that the chromosomes must in some way be involved in 
the transmission of hereditary characteristics. The proof for this idea came years 
later, and grew out of the discovery of the way in which sex is determined. For 
centuries it was believed that sex was determined by environmental forces acting 
on the embryo during its development. It would be difficult to assess the abuses 
to which mothers were subjected to ensure the production of a child of the de- 
sired sex, usually male. However, in the early 1900's it was discovered that males 
had an unequal pair of chromosomes not observed in females. The males, there- 
fore, produced two kinds of sperm, one bearing a large or X chromosome plus 
one each of the other chromosome types, the other bearing a small or Y chromo- 
some plus a set of the other chromosomes known as the autosomes. Females were 
found to carry two X's and two sets of autosomes, and their eggs after meiosis, 
one X and one set of autosomes. The X and Y chromosomes were called sex 
chromosomes because fertilization of an X-bearing egg by an X-bearing sperm 
produced a female whereas fertilization of an X-bearing egg by a Y-type sperm 
resulted in a male. Thus the cytological facts developed rapidly, but independ- 
ently of the development of knowledge about heredity. Of course cytology 
flowered late in the nineteenth century before genetics as a science even had its 
start, but even after 1900 and the rediscovery of Mendel's laws, the two sciences 
pursued independent courses. 

Sex Linkage 

Then, among the many red-eyed fruit flies in Thomas Hunt Morgan's 
laboratory at Columbia, a single white-eyed male was discovered. When crossed 
to red-eyed females, all of the F a were red-eyed. Inbreeding the F x gave a 3 red 
to 1 white ratio in the F 2 . This result seems perfectly normal, except for the fact 
that all of the F 2 white-eyed flies were males. This unusual result, it was seen, 
could be explained if the gene causing white eyes were located on the X chromo- 
some. The pattern of inheritance then would be: 

XX X Y 

Pi 

W W X ,i 

red-eyed female white-eyed male 



192 • THE MECHANISM OF EVOLUTION 



Pi gametes 
Fi 



WotW 



Ww 




or Y 



WY 



red-eyed female red-eyed male 



nFi gam 
9 X 
F x gam N. 


W 


Y 


W 


WW 
red 9 


WY 
redd" 


w 


Ww 
red 9 


wY 

white cf 



If this assumption is correct, it should be possible to predict the results 
of the reciprocal cross, white female with red male, as follows : 



Pi gametes 
Fi 



XX X Y 

\w X W 
white-eyed female red-eyed male 



WorY 



Ww 
red-eyed female 




wY 
hite-eyed male 



\Fi gam 
9 X 
Fi gam \ 


w 


Y 


W 


Ww 
red 9 


WY 
redd 1 


w 


WW 

white 9 


wy 
white cf 



White-eyed males and red-eyed females were expected in the F 1? with a 1:1 ratio 
of red and white in the F 2 , and this was the result obtained. Thus^ it seemed 
clear that the gene for white eyes must be on the X chromosome, and this un- 
usual type of inheritance, intimately associated with sex, came to be called sex 
linked. It marked the first step toward proving that all of the genes are located 
on the chromosomes, the autosomes as well as the sex chromosomes. Mendel's 
factors, then, are not mere abstractions but are physical entities borne by the 
chromosomes in the nucleus of the cell. The chromosomes are therefore the 
physical basis of heredity and of evolution. 

Though a great deal still remains to be learned, the chromosomes are 
now known to be formed of nucleoprotein, a combination of protein and 



THE PHYSICAL BASIS OF EVOLUTION • 193 

deoxyribonucleic acid, with the latter in all probability the vehicle of hereditary 
information. These small bodies, measured in thousandths of millimeters, carry 
the factors that in large measure determine not only man's outward appearance — 
his build and height, his skin, eye, and hair color — but also less obvious traits, 
such as disease resistance, intelligence, and personality. 

The discovery that the genes were located on the chromosomes opened 
up entire new areas to exploration in the search for knowledge about heredity, 
and also gave new insight into the mechanism underlying Mendel's laws. The 
separation of maternal from paternal chromosomes at meiosis is the basis of 
Mendel's first law of segregation. The random alignment of chromosome pairs 
at metaphase is the basis of Mendel's second law of independent assortment. In 
other words, the position on the metaphase plate of the maternal and paternal 
chromosomes of one chromosome pair is independent of their position in any 
other pair; hence the gametes contain random combinations of maternal and 
paternal chromosomes. As the number of chromosome pairs increases, the num- 
ber of possible kinds of gametes grows, the number of kinds doubling with each 
added pair. In man, for example, with 23 pairs of chromosomes, 2 23 different 
combinations of maternal and paternal chromosomes are possible in the gametes 
of a single individual. Small wonder that even brothers and sisters are never 
alike. 

The number of genes in any species far exceeds the number of chromo- 
some pairs. Obviously some of the different genes must reside on the same 
chromosome. In such cases, Mendel's law of independent assortment does not 
hold, for genes on the same chromosome tend to stay together in crosses, and 
are said to be linked. The discovery that the genes are on the chromosomes was 
the basis of the third major principle of heredity, the principle of linkage. How- 
ever, this linkage is not complete, for crossing over or recombination between 
genes on the same chromosome sometimes occurs. The chiasmata formed in first 
meiotic prophase are the visible evidence of the exchange of segments of chro- 
matids between maternal and paternal chromosomes, which forms the basis for 
crossing over. Hence, recombinations occur within as well as between maternal 
and paternal chromosomes, and the amount of possible recombination is in- 
creased far beyond 2 23 . 



The chromosome mechanism is the physical basis not 
only of heredity but of evolution. The factors discovered by 
Mendel are located in the chromosomes. The behavior of the 
chromosomes is responsible for Mendelian segregation and inde- 
pendent assortment. However, genes on the same chromosome 
tend to be inherited as a linked group, occasionally broken up by 



► SUMMARY 



194 • THE MECHANISM OF EVOLUTION 

crossing over. A favorable combination of genes within a chromo- 
some tends to be held together and not broken up completely in 
the next generation. Natural selection preserves favorable gene 
combinations, but could not very well do so if completely inde- 
pendent assortment of genes occurred each generation. Hence, 
even the organization of genes into chromosomes can be regarded 
as adaptive, a means of preserving favorable gene combinations; 
recombination and crossing over give rise to variations, which 
make possible adaptations to new or changing environmental 
situations. 

SUGGESTED READING 

Darlington, C. D., 1937. Recent advances in cytology, 2d ed. Philadelphia: Blak- 
iston. 

Riley, H. P., 1948. Introduction to genetics and cytogenetics. New York: Wiley. 

Swanson, C. P., 1957. Cytology and cytogenetics. Englewood Cliffs, N. J., Prentice- 
Hall. 

White, M. J. D., 1954. Animal cytology and evolution, 2d ed. Cambridge University 
Press. 



Pa 



Bl_ b£ 

Yl X ~bL 

blue round red long 



P x gam Bl bL 



IL 


back 




U_ 


Jl 


cross 


to 


Ti 


blue long 






red round 



CHAPTER 



19 



Linkage 



Linkage and Crossing Over 

Even though genes on the same chromosome tend to be 
inherited as a group, recombination or crossing over between 
linked genes does occur. The mechanism of crossing over is a 
reciprocal exchange of segments between two nonsister chroma- 
tids, which occurs in the four-strand tetrad stage of first meiotic 
prophase, and is observable cytologically as a chiasma and genet- 
ically as a recombinant or crossover phenotype. The phenomenon 
of crossing over has made it possible to map out the relationships 
between the genes on the same chromosome pair. 

Let us first examine a cross involving two pairs of linked 
genes. The first work in which linkage was recognized was carried 
out by Bateson and Punnett with the sweet pea in 1906. The 
traits were long (L) versus round (/) pollen and purple or blue 
(B) versus red (b) flowers. Crosses involving these traits gave 
the following results: 



195 



196 • THE MECHANISM OF EVOLUTION 



\ Fx 












\gam 












Fi \ 






n 


percent 




gam \^ 


bl 


phenotype 


obs. 


obs. 




Bl 


Bl 

Jl 


blue 
round 


153 


43.5 


non CO. 


bL 


bL 

Ti 


red 
long 


155 


44.1 


non CO. 


BL 


BL 

~bl 


blue 
long 


23 


6.5 


CO. 














bl 


bl 


red 
round 


21 


6.0 


z.or 




bl 









In this instance, instead of 25 percent of the total in each of the F 2 
categories expected with independent assortment, there was a great excess of the 
original parental types and a deficiency of the recombinant or crossover types. 
Rather than 50 percent new types, only 12.5 percent crossing over occurred. This 
frequency of crossing over is remarkably constant between any given pair of 
gene loci. 

Linear Order of the Genes 

Next let us consider an example involving three pairs of linked genes. 
Echinus (ec) is a recessive mutant in the fruit fly (Drosophila melanogaster) 
causing rough eyes; scute (sc), a recessive causing some bristles to be missing; 
and crossveinless (cv) eliminates the crossveins of the wings. The wild-type 
genes for all three mutants can be designated by a plus, a convention that makes 
the following cross somewhat easier to follow: 



Pi 



+ ec + sc + cv 
+ e c + sc + cv 



Pi gam 


+ e c + sc + cv 






Fi 


00+"+ back 
sc + cv cross 


to 


sc ec cv 




sc ec cv 



cTo 71 



non CO. gametes I 



>v F lC f 
\ gam 
Fi NT 

9 gam N. 


sc ec cv 


individuals 
observed 


' +ec + 


±jc± ^ 
sc ec cv 


810 


sc-\- cv 


JC+ cv 
sc ec cv 


828 






LINKAGE • 197 



CO. gametes 



sc ec + 


sc ec-\- 
sc ec cv 


62 


+ +CV 


+ +.CP 

sc ec cv 


88 


sc+ + 


sc+ + 
sc ec cv 


89 


+ ec cv 


-f- ec cv : 
sc ec cv 


103 


+ + + 


+ + + 
sc ec cv 





sc ec cv 


sc ec cv 





sc ec cv 






total 


1980 



If each gene pair were on a different pair of chromosomes, equal numbers of 
flies would have been observed in each of the eight phenotypic classes. However, 
independent assortment obviously did not occur, for the numbers range from 
to 828. The crossover percentage between two linked gene loci is determined 
by dividing the number of individuals showing recombination between these two 
loci by the total number of individuals of all types and multiplying by 100. 

C O 

percent CO. = — - X 100 

fo -I— fift -4- n -4- n 
percent CO. between sc and ec = T7 ^ X 100 = 7.6 percent 



percent CO. between ec and cv = 
percent CO. between sc and cv = 



1980 
89+103 + + 



62 + 



1980 

)S + 89 + 103 



X 100= 9.7 percent 



1980 



X 100 = 17-3 percent 



Crossover percentages between linked genes may range anywhere from very close 
to percent up to 50 percent, depending on which two genes are chosen. 
These crossover frequencies not only indicate that these genes are linked, but 
they also make it possible to arrange them in a definite linear order. This line, 
with the genes marked off at intervals determined by the crossover frequencies, 
is known as a chromosome map. From the above data, the following map can be 
constructed : 

7.6 17.3 



sc 



ec 



-7.6- 



-9.7- 



17.3- 



198 • THE MECHANISM OF EVOLUTION 

No individuals appeared at all in the two double crossover classes, +++ 
and sc ec cv. If crossovers in the two regions sc-ec and ec-cv were independent 
events, the expected probability of simultaneous or double crossovers in these 
regions would be 7.6 percent X 9.7 percent == 0.7 percent. In other words, 
about 14 double crossover individuals would have been expected in this cross, 
but none was observed. Therefore, it appears that if one crossover occurs, the 
probability of another crossover in adjacent regions of the same chromosome is 
reduced. This phenomenon, known as interference, indicates that crossing over 
must involve segments of the chromatids rather than individual gene loci. Inter- 
ference is complete, as in this case, within a certain distance from the first cross- 
over, and becomes progressively less the farther away the second crossover is 
from the first. The proportion of expected double crossovers that actually occur 
is called the coincidence, which thus serves as an indication of the amount of 
interference. 

Actually, the only satisfactory way to represent the relationships of 
linked genes graphically is to show the genes as points on a line. In numerous 
linkage tests made with a variety of species, if the crossover frequencies, say for 
three gene loci a, b, and c, are ab and be, then the frequency of ac is either ab 
plus be, as in the example above, or ab minus be if c lies between a and b. 
Results such as these form the basis of the fourth and final major principle of 
genetics, the linear order of the genes. Of the four principles, Mendel was re- 
sponsible for segregation and independent assortment, and Morgan and his co- 
workers for linkage and the linear order of the genes. 

Extending these test crosses makes possible a complete mapping of each 
chromosome. There are only as many linkage groups as there are chromosome 
pairs, and each gene can be located with respect to all of the others. The greater 
the physical distance between two genes on the same chromosome, the greater 
the chance of recombination between them, and the farther apart they will ap- 
pear on the map. 



SUMMARY <■ 



The genes, the basic units of evolution, are located on 
the chromosomes and are arranged in a linear order that can be 
mapped with considerable precision. Evolution, therefore, occurs 
within the limits imposed by the chromosome mechanism of 
heredity. 



SUGGESTED READING 

See references at the end of Chapter 16. 



CHAPTER 



20 

Chromosomal Variation 



Linkage studies and chromosome mapping are possible 
because the structure of the chromosomes is very stable. On rare 
occasions, however, chromosome rearrangements may occur. These 
rearrangements can usually be detected both cytologically and 
genetically, for the linkage relationships of the genes are changed 
by any restructuring of the chromosomes. In order for rearrange- 
ments to occur, the chromosomes must break. Chromosome break- 
age may be "spontaneous," but it can also be induced by such 
agents as ionizing radiation and certain chemical compounds. In 
many cases the breaks heal or restitute with no detectable cytolog- 
ical or genetic effect. However, if the broken ends fail to unite or 
else reunite in new combinations, they then can be detected. 

Duplication and Deficiency 

A number of types of rearrangements have been recog- 
nized (see Fig. 20-1). A deficiency or deletion may arise as 
follows : 

ABC DEFGH -4 ABC DE+FGH 

• • 

T 

breakage deficiency for acentric 

point FGH region fragment 

A deficiency is often lethal when homozygous, or even, if large 
enough, when heterozygous, and is therefore not apt to play a 
role in evolution. 



199 



200 • THE MECHANISM OF EVOLUTION 




DELETION 
of c - d region 




8 





In synapsis 



INVERSION 
1 of c-d region 




DUPLICATION 
of c - d portion 




synapsis 




In synapsis 





TRANSLOCATION 
between nonhomologous 
chromosomes 



n synapsis 



Fig. 20-1. Types of chromosome rearrangements. 



way: 



CHROMOSOMAL VARIATION • 201 
A duplication of a chromosome segment may arise in the following 



ABC DEFGH-* ABC DEFEFGH 

• • 



ABC DEFGH 

• 

T T 



duplication for 
EF region 



The addition of the extra EF segment gives rise to a duplication or repeat of that 
region. Duplications are generally viable and represent a way of adding addi- 
tional gene loci to the genotype. Furthermore, it has been suggested that muta- 
tion can then produce genes of divergent function as follows : 



duplication 



E 



mutation 



E' 



In this way, during the course of evolution the total number of genes could be 
increased with a corresponding diversity of function. 

Inversion 

An inversion results when two breaks in a chromosome rejoin after the 
fragment has rotated 180 degrees. 

ABC DEFGH -» ABC DGFEH 
\ n i inversion 

The linkage relations are changed with G, for example, now closely linked with 
D rather than H. Inversions that include the centromere are pericentric; those 
not including the centromere are paracentric. Individuals may be either homo- 
zygous or heterozygous for an inversion. In inversion heterozygotes, the synapsis 
of homologous chromosomes at meiosis is somewhat abnormal, for homologous 
genes continue to pair wherever possible despite their different linkage relations 
in the two homologues. As a result of these pairing forces the chromosomes are 
thrown into easily recognized, characteristic loops. If pairing and crossing over 
do occur, abnormal chromosomes and fragments are frequently produced that 
are usually unviable. Hence, the inversions act essentially as crossover suppressors, 
preventing recombination within chromosomes since the crossover products give 




202 • THE MECHANISM OF EVOLUTION 

rise to gametes with aberrant haploid sets of chromosomes for the most part. 
Thus in an evolutionary sense inversions are conservative because ordinarily only 
the old gene combinations give rise to viable organisms. 



Translocation 

A reciprocal translocation arises when breaks in two chromosomes are 
followed by reunion with the fragments interchanged. 

ABC D E N M 

• 



HGFOPQ RST 

— •— ■ • 

T 

Genes in the exchanged fragments now belong to new linkage groups, but the 
genes will still pair with their old allelic partners so that in a translocation 
heterozygote four chromosomes will form a single synaptic figure. 



ABC 


D E F G 


H 


• 
M N O P 


T 

Q RST 

• 



H 



IV 



ABC 


D E 


a 


ABC 


D E 


III 


N 
M 



H 



O P 


Q 


RST 




* 




O P 


Q 


RST 


N 
M 




II 



A little study will show that if chromosomes I and IV go to the same 
pole, the gametes will be deficient for the genes in the region MN while those 
in region FGH will be duplicated. The reverse is true if chromosomes II and III 
go to the same pole. Because of the deficiencies, sterility will ensue. Only combi- 
nations of I and III or II and IV can be expected to be fertile. Furthermore, 
crossing over may lead to additional sterility. If several translocations are present, 
rings of chromosomes, chains of chromosomes, or other unusual synaptic con- 
figurations will be observed in meiotic prophase because of the specificity of the 
pairing reaction. Crosses between populations having different gene arrange- 
ments, whether inversions or translocations, will not ordinarily be selectively 
advantageous since there is partial sterility in the resulting progeny. In some 
species, however, inversions (for example, Drosophila) and translocations (for 



CHROMOSOMAL VARIATION • 203 

example, Oenothera) have become a part of the normal genetic system within 
breeding populations, apparently having an adaptive function. 



Position Effect and Pseudoallelism 

In addition to changing the linkage relationships, in some cases re- 
arranging the relationships of the genes to each other changes their effects on the 
phenotype though the genes themselves are apparently unchanged. This phe- 
nomenon is known as position effect. The classical example of position effect 
involves Bar eye in the fruit fly. The Bar-eye condition is due to the duplication 
of a small segment of the chromosome and can be diagramed as follows: 



1. wild type 



2. Bar eye 



3. double Bar 



[] 



4. double Bar/ 
wild heterozygote 



The genie contents of types 2 and 4 are identical, but the heterozygote 
has significantly smaller eyes than the homozygous type. Hence, the phenotypic 
difference must be due to the genes' arrangement, and the expression of a gene 
is dependent not only on its intrinsic effects but also on its position with respect 
to the other genes in the genotype. 

Position effect has also been found to be the rule with pseudoalleles. 
The term pseudoallele was coined to describe cases originally thought to involve 
a single locus with multiple alleles but that turned out to be two or more very 
closely linked loci with all the genes affecting the same trait. One interpretation 
currently favored is that these loci arose by duplication (hence their similarity in 
action) followed by mutation to divergent functions as suggested above. The 
white-apricot case in Drosophila melanogaster will serve as an example of posi- 
tion pseudoallelism. The eye-color mutants, white and apricot, were originally 
thought to be members of a multiple allelic series at the white locus, and were 
designated w and w & . The discovery of rare crossovers (approximately 0.01 per- 
cent) between white and apricot indicated that separate closely linked loci were 
involved, and the mutants were designated w and apr. Position effect was re- 
vealed when the phenotypes of the two kinds of double heterozygotes were 



204 • THE MECHANISM OF EVOLUTION 

compared. In the cis condition both mutant genes are on one chromosome, both 
wild-type genes on the other. The trans state has one mutant and one wild-type 
gene on each homologue. 

apr w apr + 



+ + + w 

cis trans 

The cis phase has phenotypically wild-type red eyes whereas the trans has a 
light apricot eye color. Since both types of double heterozygotes have exactly the 
same genes, position effect is obviously involved. The discovery of position effect 
and of pseudoallelism has led to a considerable revision in the gene concept. 

Heteroploidy 

Let us now consider chromosomal variations involving changes in the 
numbers of whole chromosomes rather than rearrangements involving chromo- 
some fragments. Two general types of change have been found. Polyploids (or 
euploids) are individuals with one or more complete haploid sets of chromo- 
somes added to the usual diploid number. Heteroploids (or aneuploids) have 
some number of chromosomes other than an exact multiple of the haploid 
number. 

A heteroploid, for example, may have an extra chromosome from one 
pair, or In + 1 chromosomes, and is then known as a simple trisomic. If a 
chromosome from one pair is lacking (2n — 1), it is known as a simple 
monosomic. These and more complex heteroploids tend to lead to sterility or 
deficient gametes, and hence are generally of little evolutionary significance. 

Polyploidy 

The changes involving whole haploid sets of chromosomes, however, 
have been of considerable evolutionary significance, especially in plants. These 
polyploids may be of several kinds, among the more common being triploids 
(3«), tetraploids (4/z), hexaploids (6n), and octoploids (8;?). Many domesti- 
cated plant species are polyploid (wheat, cotton, apples, etc.), and it is now 
possible for plant breeders to induce polyploidy with colchicine, a chemical sub- 
stance that inhibits the formation of the mitotic spindle. The polyploids fre- 
quently have more vigorous vegetative growth and larger and more intensely 
colored flowers, and hence are especially desirable as new horticultural varieties. 

Polyploidy arises in two distinctly different ways. A multiplication of the 
chromosome sets from a single species gives rise to autopolyploidy. If A, for 
instance, represents a single haploid set of chromosomes, the diploid will be AA, 



CHROMOSOMAL VARIATION • 205 

and an autotetraploid, AAAA. Though vegetative vigor is usually good, sterility 
is high in autopolyploids due to abnormal synapsis at meiosis when more than 
two homologous chromosomes form a synaptic figure. 

Allopolyploids or amphiploids are formed when hybridization between 
two different species is followed by a doubling of the chromosome number in the 
diploid hybrid or by the formation of unreduced gametes: 



p. 


AA X BB 


P t gametes 


A B 


F x 


AB 




chromosome doubling 


F x gametes 


AB X AB 


F 2 


AA BB allotetraploid 



The Fi AB hybrid is generally quite sterile due to the lack of pairing 
between the chromosomes of the A and B genomes. The F 2 allotetraploid, on 
the other hand, is fertile, acting as a functional diploid, since each type of A 
and B chromosome is represented twice, and pairing at meiosis is normal be- 
tween these homologues. In some cases polyploids more or less intermediate to 
the auto- and allopolyploids have been formed, which are known as segmental 
allopolyploids. 

More than one third of all species of higher plants, the angiosperms, 
are polyploid, and thus polyploidy has been of considerable importance to plant 
evolution. With the discovery of means of inducing polyploidy, new horizons 
have been opened to the plant breeders. An early and classical example of a 
synthetic allotetraploid was Rap ban o bras ska, formed from the radish {Raphanus) 
and the cabbage (Brassica). Such a plant obviously had considerable potential 
since the edible portions are the root in one parent, the shoot in the other. 
Briefly, the details of the cross are as follows: 

p. 



radish X 


cabbage 


2;? x = 18 


2n 2 - 18 


n x - 9 


n 2 = 9 


»1 + » 2 = 


= 18 



Pi gametes 

sterile diploid 

chromosome doubling 
¥ 1 gametes («! + n 2 ) X (n 1 + n 2 ) 

F 2 n x n x n 2 n 2 

fertile allotetraploid 

One difficulty emerged when these sturdy, fertile F 2 plants were examined; they 
had a root like a cabbage and a head like a radish. 



206 • THE MECHANISM OF EVOLUTION 

SUMMARY A 



Chromosomal variation as well as genie variation can be 
observed in natural populations. These variations include re- 
arrangements involving chromosome fragments such as duplica- 
tions and deficiencies, inversions, and translocations. The addition 
or loss of whole chromosomes gives rise to heteroploidy, in which 
the number of chromosomes does not equal an exact multiple of 
the haploid number. Polyploids, with additional complete haploid 
sets of chromosomes, may arise within a single species or subse- 
quent to hybridization between different species. Chromosomal 
rearrangements may, on occasion, lead to position effects when the 
gene, in a new location with respect to the rest of the genes, has 
a changed effect on the phenotype even though the gene itself is 
apparently unchanged. 



SUGGESTED READING 

See references at the end of Chapter 18. 






CHAPTER 



21 



Mutation 



Over a century ago a short-legged ram unlike any of the 
other sheep was born into the flock of a New England farmer 
named Seth Wright. This ram transmitted the short legs to his 
progeny, and from him was thus derived the Ancon breed of 
sheep (see Fig. 21-1), valued by New Englanders because these 
sheep were unable to jump the stone fences so common there. 
Apparently they were not prized for very long, since the breed 
became extinct about eighty years ago. However, more recently a 
Norwegian lamb with short legs appeared, and from this animal 
a new strain has been developed. The s^ddejn^iLp^aianjce^oi-a 
new hereditary trait in a population is said to be due to a muta- 
tion^a change in the hereditary jnaterial. In this case, the traj JL 
behaved as a simple recessive in crosses, and presumably had 
its ^origin by mutation and^ not by the recombination of existing 
genes. A great variety of mutations has been observed in a number 
of different species. The valuable platinum mutation in the fox, 
streptomycin resistance in bacteria, and the hemophilia mutation 
("bleeder's disease") that Queen Victoria bestowed so liberally 
among her descendants are cases in point. 

Types of Mutations 

In a broad sense a mutation is any hereditary change not 
due to the simple recombination of genes. Included in this sense 
are gene or point mutations, chromosomal changes, either struc- 
tural or numerical, and position effects. In a narrower sense, muta- 



207 



208 • THE MECHANISM OF EVOLUTION 

tion is used to refer to a self-duplicating change at a single gene locus. Gene 
mutations are of fundamental importance to evolution because they form the raw 
material of evolution. Only by mutation can truly new kinds of genetic variation 
appear, and all evolutionary change is based, ultimately, on mutation. Mutation 
alone, however, cannot account for evolution, for the sporadic mutants must in 
some way become a part of the genotype of the population. 

There is no simple method of classifying mutations, for they may affect 
all kinds of traits in the organism, from its pigmentation to its psychoses, and 




Fig. 21-1. Normal ewe on left. Short-legged Ancon ewe in the center and ram 
on the right are homozygous for the recessive Ancon mutation. 



they are therefore of an almost bewildering variety. One method of classification 
frequently used takes only the effect on viability into consideration, and the 
mutants are then classified as lethal, semilethal, subvital, normal, and supervital. 
Another common approach is to group the mutations according to their visible 
effects on the phenotype, and mutants are described as wing mutants, eye-color 
mutants, body-color mutants, bristle mutants, etc. However, the so-called "white- 
eye" mutant in the fruit fly also causes transparency of the testicular envelope, a 
change in spermatheca shape, and a lowered viability, longevity, and fertility. 



MUTATION • 209 

Hence, to call white an eye-color mutant scarcely indicates the entire story. These 
genes with a multiplicity of effects are said to be pleiotropic, but the apparent 
variety of effects may be traceable to a single primary change in gene function. 
The observed phenotypic effects are generally far removed from the primary 
action of the gene. The biochemical mutants in microorganisms may be some- 
what closer to the primary gene action. These mutant types usually fail to form 
a particular biochemical substance such as an amino acid or a vitamin because of 
the absence or inactivation of an enzyme needed to mediate the synthesis. Study 
of mutants of this type may in time do away with the need for the more or less 
arbitrary classifications of mutants currently in use. 

Induced Mutation 

"Spontaneous" mutations occur all the time, but they are called "spon- 
taneous" simply because the exact causes are not as yet well understood. The 
mutation rate can be raised well above this "spontaneous" rate by various experi- 
mental techniques that have provided some insight into the mechanisms of 
mutation. Temperature shocks were one of the first methods used to raise the 
mutation rates; in flies, exposures for short periods to both low and high tem- 
perature extremes outside the normal range were found to be effective. Within 
the normal temperature range of the organism, mutation rates will be higher at 
the higher temperatures. 

The discovery that x-rays and other ionizing radiations (a, f3, and y 
rays, protons, neutrons) induced mutations and caused chromosome breakage 
marked a milestone in the study of mutation. The number of mutations is directly 
proportional to the dose of radiation and is independent of intensity. In other 
words, a dose of 500 roentgens (a roentgen or r unit produces two ionizations 
per cubic micron of tissue) will cause the same number of mutations whether 
received over a period of 20 minutes or 20 months, and the effect is cumulative. 
Chromosome breaks are presumed to be proportional to dose also. However, 
two-hit chromosomal aberrations (for example, translocations, whose formation 
depends on the simultaneous occurrence of two open breaks) show an intensity 
effect, since at low intensities one break usually reunites before another break 
occurs. Ultraviolet light, essentially a nonionizing radiation, is also mutagenic 
though relatively less effective at breaking chromosomes than the ionizing 
radiations. 

The mutagenic properties of the mustard gases were discovered during 
World War II, and since then a variety of chemical substances has been shown 
capable of raising rates of mutation and chromosome breakage. As yet, no pattern 
is apparent in the types of effective compounds, which include peroxides, for- 
maldehyde, urethane, triazine, diepoxide, caffeine, phenol, and also cancer- 
producing compounds such as dibenzanthracene and methyl-cholanthrene. 



210 • THE MECHANISM OF EVOLUTION 

Study of the effects of mutagenic agents in combination with each other 
or with other agents has shown a variety of modifying effects. Infrared alone is 
not mutagenic, but pretreatment with infrared followed by x-radiation raises the 
yield of aberrations above that of the same dose of x-rays alone. On the other 
hand, exposure of cells to ionizing radiations under conditions of anoxia gen- 
erally reduces the yield of aberrations as compared to radiation with oxygen 
present. The mutagenic effects of ultraviolet light can be counteracted by subse- 
quent exposure to visible white light. Chemical substances such as reducing com- 
pounds, British anti-Lewisite (BAL), and alcohol have been shown to protect 
cells against radiation damage. However, even though such findings offer the 
hope that some protective measures can eventually be developed against the 
physiological and genetic damage caused by atomic warfare or other radiation 
hazards, the therapeutic consumption of large quantities of alcohol in the event 
of an atomic war has not yet been recommended. 

Mutation Rates 

A most interesting aspect of the mutation process was revealed by the 
discovery of the so-called mutation-rate genes, which affect the mutation rates of 
genes at other loci. In corn, for example, the recessive a x gene (the A x locus con- 
trolling anthocyanin production) is stable in the presence of the recessive dt 
allele at the dotted locus. The dominant Dt, however, induces instability in the 
a x allele, causing it to mutate to A x at a high rate, so high, in fact, that it is 
called an "ever-sporting" gene. (New mutant types used to be called "sports" 
before the term mutation came into general use. It seems a pity, almost, that the 
more colorful word was not retained.) Another instance is the "hi" mutant in 
Drosophila, which differs from Dt in that it raises mutation rates at many loci 
rather than just one, and also induces chromosome breakage. The existence of 
these mutation-rate genes raises the intriguing possibility that the mutation rates 
in natural populations can be controlled by natural selection by either favoring or 
eliminating these genes. 

Mutation is essentially a random process in that it is not possible to 
predict when a given gene will mutate, nor do mutations occur as an adaptive 
response to an environmental stimulus. However, it is not completely random, 
for the mutations occur within the framework of the existing genotype. Further- 
more, the same mutation tends to recur, time and again, but different rates of 
mutation prevail at different loci and for different mutational changes at the 
same locus. Hence, all types of mutations do not have the same probability of 
occurrence and some genes are more stable than others, but all of them, except 
the ever-sporting variety, are exceedingly stable. In man, for instance, the muta- 
tion rate to the dominant gene causing aniridia, absence of the iris, has been esti- 
mated at 10 per million gametes or 1/100,000. One way to consider this fact is 



MUTATION • 211 

that a single normal allele would be expected to go through 100,000 generations, 
on the average, before it mutated. Another way, however, equally valid, is to 
state that a single ejaculate containing 100,000,000 spermatozoa would be ex- 
pected to contain approximately 1000 sperm cells carrying new aniridia mutants. 
The mutation rate from the normal condition to the sex-linked recessive gene 
causing hemophilia has been estimated at one in 31,000 gametes; that to the 
autosomal dominant causing achondroplastic dwarfism is approximately one in 
24,000. 

In corn, more precise studies than in the human material have shown a 
wide range of spontaneous mutation rates, as given below : 







gametes 


number of 


average per 
million 


trait 


mutation 


tested 


mutations 


gametes 


colored— >noncolored aleurone and plant 


R-*r 


554,786 


273 


492 


inhibitor—* noninhibitor of aleurone color 


I-*i 


265,391 


28 


106 


purple—* red aleurone 


Pr-^pr 


647,102 


7 


11 


starchy — > sugary endosperm 


Su—> su 


1,678,736 


4 


2.4 


yellow — * white starch in endosperm 


Y-+y 


1,745,280 


4 


2.2 


full — > shrunken endosperm 


Sh-+sh 


2,469,285 


3 


1.2 



non waxy — > waxy endosperm Wx—+wx 1,503,744 

These figures may be compared with those given above for man : 

average tier 





average per 


trait 


million gametes 


achondroplasia 


42 


hemophilia 


32 


aniridia 


10 



It will be seen that the rates per generation are roughly of the same 
order of magnitude even though the generation lengths are quite different. The 
same is true of bacteria and Drosophila with even shorter generation lengths. 
The fact that species with generation lengths ranging from about half an hour 
to thirty years have comparable average mutation rates per locus per generation 
of roughly 10" 5 to 10" 6 seems to bear out the earlier suggestion that mutation 
rates are to some extent under the control of natural selection. If, on an absolute 
time basis, the bacterial mutation rates prevailed in man, the human load of 
mutations would be enormous. 

Most of the mutations that occur are deleterious and recessive to the 
prevailing types of genes. These genes, the "wild type," are the favorable muta- 
tions of the past, which have been preserved by natural selection and have in- 
creased in frequency until they have become the most frequent type. Thus, any 
random change affecting these favorable genes has a much greater probability of 
being deleterious than it has of being more favorable than the existing genes. 

Though Bateson and Punnett at first visualized recessive mutations as 
complete losses or deficiencies of the gene loci, the discovery of back mutations 



212 • THE MECHANISM OF EVOLUTION 

has made this idea untenable. Even though many apparent reverse mutations 
have turned out, on careful genetic analysis, to be due to mutations at entirely 
different loci, nevertheless, careful analyses such as those of Giles with Neuro- 
spora have established the existence of true reverse mutations. 



Controlled Genetic Changes 

None of the mutagenic agents discussed thus far can be used to induce 
a predictable specific mutation. Present techniques, both radiation and chemical, 
involve essentially a shotgun treatment, with the geneticist examining the pieces 
for whatever mutations may have occurred. This method, of course, must be re- 
garded as very crude, and it would be highly desirable, especially for the prac- 
tical breeder, if he were able to control the mutation process and to induce spe- 
cific kinds of mutations at will. At least one type of experiment has given reason 
for hope that controlled mutations may one day be possible. 

In the Pneumococcus bacteria various types have been identified that 
differ in the type of polysaccharide capsule enclosing the cell. The encapsulated 
bacteria form a smooth colony when cultured. By mutation, the ability to form 
the polysaccharide capsule may be lost, and the unencapsulated cells then form a 
rough colony. Back mutation will give rise to encapsulated cells, but the capsule 
always has the same type of polysaccharide as the original type. For example, 

smooth mutation rou g h mutation smooth 

Type I > Type I > Type I 

However, the addition of an extract from killed bacteria with a different capsular 
type produced the following result: 

smooth mutation rou g h extract from ^ smooth 

Type I > Type I Type III Type III 

In this case, a predictable change was induced, but the active inducing agent was 
not the Type III polysaccharide itself, but rather the DNA (desoxy ribonucleic 
acid) from the Type III bacteria. Bacterial transformation, as this phenomenon 
is called, may not represent a true induced mutation, but it is an induced directed 
hereditary change, and hence is extremely significant as a step toward directed 
mutation. 

In a somewhat similar case known as transduction, genetic material can 
be transferred from one bacterial strain to another via a bacterial virus. The virus 
apparently transports the genes or a small chromosome segment from one bac- 
terial host to another where it becomes incorporated into the genotype of the 
new host. 



MUTATION • 213 








Fig. 21-2. Some "mutants" of the evening primrose, Oenothera lamarckiana, on 

which de Vries based his mutation theory. Oenothera lamarckiana above. The 

"mutants" from the left counterclockwise are: O. gigas, O. albida, O. scintillans, 

and O. oblonga. (From de Vries.) 



The Mutation Theory of de Vries 

In the very early days of genetics de Vries (1902) proposed the muta- 
tion theory of evolution as an alternative to the theory of natural selection, de 
Vries had been working with the evening primrose, Oenothera lamarckiana, in 
which new and strikingly different types of plants occasionally appeared, breed- 
ing true to the new type (see Fig. 21-2). On the basis of this work, de Vries 



214 • THE MECHANISM OF EVOLUTION 

suggested that new species originate as a result of these large discontinuous 
variations or mutations rather than from the gradual accumulation of numerous 
small hereditary differences in size, shape, color, etc., by natural selection. How- 
ever, his theory turned out to be based on a variety of changes, stemming from 
the unique features of the genome of Oenothera, and including tetraploidy, 
trisomies, reciprocal translocations, and balanced lethal systems. With a few pos- 
sible exceptions, these hereditary changes did not represent genie mutations at all 
even though they bred true and remained distinct from the parental types; rather, 
they were actually the result of recombination of chromosomes or genes. These 
spurious mutants in Oenothera are the result of a unique situation not to be 
found in all species, and therefore they cannot serve as a general mechanism for 
evolution. 

As the knowledge of heredity has increased, mutations of all degrees 
have been studied. Their effects may be great, or they may be so small that re- 
fined statistical or genetic methods are needed to detect the difference between 
different mutant types. As an understanding of the nature of mutation has de- 
veloped, it has become clear that de Vries, though basing his mutation theory of 
evolution on changes that were not genie mutations at all, was fundamentally 
correct in stressing the significance of mutation to the evolutionary process. 
However, mutation alone cannot account for evolution; rather it furnishes the 
raw materials on which other forces act to bring about evolutionary change. 



SUMMARY < 



In a broad sense mutation implies a change that takes 
place in the hereditary material and does not arise as a conse- 
quence of recombination. In a narrower sense mutation is used to 
refer to a self-duplicating change at a specific locus. Mutations 
form the raw working material of evolution, for the mutation 
process is the only one giving rise to entirely new kinds of 
hereditary variation. Because spontaneous mutations are typically 
recurrent, it is possible to estimate mutation rates. These rates may 
be increased by various treatments such as temperature shock, 
ionizing radiations, and chemical mutagens, and by the effects of 
mutation-rate genes. Mutation is a random process in the sense 
that it is impossible to predict when a given gene will mutate and 
that mutations do not occur as adaptive responses to environ- 
mental stimuli. However, they can only occur within the frame- 
work imposed by the existing genotype. Most new mutants are 
deleterious, presumably because the prevailing "wild types" are 
the favorable mutations of the past, preserved by natural selection, 
and any random change in these favorable genes has a greater 
chance of being harmful than of having increased adaptive value. 



MUTATION • 215 

The mutation theory of evolution, suggested by de Vries as an 
alternative to natural selection, is not sufficient alone to account 
for evolution, but mutation and natural selection together are 
major factors in evolution. 

SUGGESTED READING 

Demerec, M., ed., 1951. "Genes and mutations," Cold Spring Harbor Symp. Quant. 

Biol., Vol. 16. Long Island Biological Ass'n, New York. 
Muller, H. J., 1959. "The mutation theory re-examined," Proc. X International 

Congress of Genetics, i;306-317. 
Stadler, L. J., 1954. "The gene," Science, 120:81 1-819. 



CHAPTER 



22 

Quantitative Inheritance 



Thus far, the traits we have considered have been dis- 
continuous, and the differences have been qualitative and could 
be easily determined. A person is either red-haired or he is not 
Classifying people according to height or weight is something 
else, for they are not just tall or short, thin or fat; they fall into a 
continuous pattern from tall to short, thin to fat. In fact, more 
people fall into the intermediate height and weight ranges than 
at the extremes. They must be measured rather than classified, and 
the frequency distribution of these measurements takes the form 
of a bell-shaped normal curve. When, for example, the height of 
a group of college men was measured, the frequency distribution 
had the form shown in Fig. 22-1. 

Such a population can be described in terms of the mean 
and the standard deviation. The mean or average falls at the 
center of the normal curve, and is estimated from the sample as 

n 
where x = mean 

S = the sum of 

x = the measurement on one individual 

n = the number of individuals measured 

The standard deviation (s) is a measure of the variability of the 
group and is computed as 



_ IK* 



x — at) 5 



216 



QUANTITATIVE INHERITANCE • 217 

More than 99 percent of the individuals in the population should fall within 
plus or minus three standard deviations from the mean. The standard deviation 
thus provides a way of comparing an individual with the population of which 
he is a part. The square of the standard deviation (s 2 ) is of considerable 
theoretical importance in the study of variability and is known as the variance. 
The standard error of the mean (s$) is estimated as 



Sx = 



y/ n 



and is useful as an estimate of the variability of sample means in much the same 
way that the standard deviation is an estimate of the variability of individuals in 
a sample. 









,..•••••••. 












< 


• 


•• 








/ 






• 
• 








• 














* 








• 
• 




< 


•• 

• 

• 

> 
» 






• 




• 


















• 














/ 










• 
• 
• 




• 














• 














• 
• 










• •• 




• 

.• 
•• • 










• 

• 

• 

• 

• 

• 

• 




• • 










•v.. 

• — 





25 



20 



c 

0) 
^ 10 



5* 



Ht in inches 58 59 60 
n 1 



62 63 64 65 66 67 68 69 70 
5 7 7 22 25 26 27 17 11 



72 73 74 75 76 
4 4 1 



Fig. 22-1. The normal curve. Height in man. (Data from Blakeslee.) 

For the data from Fig. 22-1 

x = 67-31 inches 

s = 3.09 s 2 = 9.56 

s s = 0.23 

Thus, more than 99 percent of the individuals in the sample would be expected 
to have heights lying within the limits 67.31 ±5s or from 58.04 to 76.58 inches, 
and actually only 1 in 175 lies just outside this range. Similarly, more than 
99 percent of the means of comparable samples would be expected to fall within 
the limits 67.31 ±3s s or from 66.62 to 68.00 inches. 

From the normal curve, it can be seen that small deviations from the 
mean are more frequent than large, that negative deviations are as frequent as 



218 • THE MECHANISM OF EVOLUTION 

positive, and that very large deviations are not due to chance alone. An exces- 
sively fat boy, then, may be suffering from thyroid trouble, or simply, like Mr. 
Pickwick's Joe, from overeating. Thus, quantitative traits are subject to environ- 
mental modification, much more so than qualitative traits such as red hair. 

Genetics of Quantitative Traits 

The genetic analysis of quantitative traits is difficult because of their 
continuous nature and the effects of the environment, and for some time it was 
felt that a Mendelian explanation was inadequate to account for the results from 
crosses involving such traits. In a classical cross by East, for example, between 
Black Mexican sweet corn and Tom Thumb popcorn, the ¥ 1 mean was interme- 
diate between the means of the parents. The F 2 mean was similar to the F x 
mean, but the F 2 was considerably more variable than either the F x or the 
parents, the more extreme F 2 individuals overlapping the parents (Fig. 22-2). 

East and Nilsson-Ehle independently arrived at a Mendelian explana- 
tion for such results. The intermediacy of the Fj had long been interpreted to 
indicate some type of blending inheritance, but blending failed to account for 
the increased variability of the F 2 . The multiple factor hypothesis postulated that 
quantitative traits were due to the action of a number of different gene pairs, 
each cumulative but of small effect as compared to environmental influences. 
The intermediate F x was due to a partial or complete lack of dominance. The 
increased variability of the F 2 was due to the segregation and recombination of 
the many gene pairs. For instance, the above cross can be outlined as follows: 

Pi AABBCCDD X aabbccdd 

Black Mexican Tom Thumb 

1 
Fi AaBbCcDd 

frequency 
I (possible distinct genotypes 

plus genes and phenotypes) 

F 2 8 AABBCCDD 1 

7 AABBCCDd, AABBCcDD, etc. 4 

6 AABBCCdd, AABBCcDd, etc. 10 

5 AABBCcdd, AABbCcDd, etc. 16 

4 AABBccdd, AaBbCcDd, etc. 19 

3 AABbccdd, AaBbCcdd, etc. 16 

2 AAbbccdd, AaBbccdd, etc. 10 

1 Aabbccdd, aaBbccdd, etc. 4 

aabbccdd 1 

This theory, though simplified, has been very serviceable for work with quantita- 
tive traits. Some of the more obvious oversimplifications are that the genes have 



.45 
.40 H 
.35 
.30 



O- -25^ 



| .20 
B 

cL .15 H 



QUANTITATIVE INHERITANCE • 219 
.45 




.10 H 

.05 



5 6 7 8 
Ear length in cm 
TOM THUMB POPCORN (60) 
.30 



13 



14 15 16 17 18 19 20 21 
Ear length in cm 
BLACK MEXICAN SWEET CORN (54) 




10 11 12 13 

Ear length in cm 

Ft GENERATION (60x54) 



16 17 




10 



16 17 18 19 



12 13 14 15 
Ear length in cm 
F 2 GENERATION 

Fig. 22-2. Quantitative inheritance in maize. Ear length in parents, F 1 and F 2 

generations of a cross between Tom Thumb popcorn and Black Mexican sweet 

corn. (Data from East and Hayes.) 



220 • THE MECHANISM OF EVOLUTION 

equal and additive effects. Evidence is available that multiple factors, also called 
polygenes, are not all equivalent in their effects on a given trait and that the 
effect of a given genie substitution will vary with different genetic backgrounds 
rather than being simply additive. Hence, contrary to the multiple factor hypo- 
thesis, these genes are neither equal nor additive in their effects. The genetic 
situation is obviously complex, and the environmental influences on quantitative 
traits also make this type of trait difficult to study. However, such studies are 
very significant both to the student of evolution and to the practical breeder, for 
the more important economic traits and species differences have both turned out 
to be of this type. The radish, of the genus Raphanus, and the cabbage, of the 
genus Brassica, are not only distinct species but belong to different genera. When 
they have been crossed, the leaves, flowers, seed pods, etc., are intermediate be- 
tween those of the parent species, indicating differences at many gene loci of the 
multiple factor type. Sumner obtained similar results in work with two subspecies 
of the deer mouse, Peromyscus polionotus. The extent of the pigmented area 
varies considerably between the subspecies leucocephalus as compared to polio- 
notus, and crosses revealed the following situation : 

Pi leucocephalus X polionotus 

45.5 | 93.0 

F x 68.3 

1 , 
F 2 69.1 

The F 2 was more variable than the F 1} the typical result in multiple factor 
crosses. These few examples, to -which the mule could be added, illustrate a 
principle that is generally true: where crosses between members of different 
taxonomic groups are possible, the progeny are intermediate for most traits — an 
indication that evolution has proceeded by the gradual accumulation of numerous 
genetic differences. 

Multiple factors play a somewhat different type of role when they 
modify the expression of a gene of major effect. In the familiar black and white 
spotted Holstein dairy cattle, one gene locus controls spotting. SS and 5j- indi- 
viduals are self-colored; ss are spotted. However, the amount of spotting is influ- 
enced by numerous other modifying factors. These genes are detectable only in 
ss individuals and have no other known effect than their ability to modify the 
expression of the ss genotype. They are so numerous that they cannot be indi- 
vidually identified or handled genetically, yet selection by the breeder can either 
increase or decrease the amount of spotting. 

Heterosis 

The American farmer in recent years has planted hybrid corn almost 
exclusively. This hybrid corn, because of its greater sturdiness, size, and yield, is 



QUANTITATIVE INHERITANCE • 221 

of greater economic value than the varieties grown forty years ago. Hybrids fre- 
quently show such hybrid vigor, or heterosis, which in some way is related to 
their increased heterozygosity. Hybrid vigor is now being exploited in hogs, 
chickens, and other species of plants and animals. In addition to its importance 
in breeding, the heterosis phenomenon, which is a special aspect of quantitative 
inheritance, plays a role in evolution. See Fig. 22-3. 

Let us consider a representative case of heterosis. Corn, which is usually 
cross-pollinated, can be self-fertilized to produce inbred lines, each very uniform, 
of poor quality, and distinct from the others. A cross of two inbreds gives an 
F x hybrid of greatly increased size and yield. The F 1} rather than being inter- 
mediate between the inbred parents, has a considerably greater yield because of 
the larger plants with more ears per stalk, more rows per ear, and more kernels 
of larger size per row. However, this heterosis cannot be perpetuated, for the 
yield in the F 2 , F 3 , and subsequent generations becomes progressively less with 
the inbreeding of each generation until, by the F 7 or F 8 , the vigor is down to 
the level of the original inbred parents. 

Two major theories have been proposed to explain the origin of 
heterosis. Both are Mendelian, variations of the multiple factor hypothesis; one 
is known briefly as the dominance theory, the other as overdominance. In 1917, 
D. F. Jones proposed the theory of linked favorable dominant genes to account 
for heterosis. He assumed that the genes favoring increased vigor, yield, size, etc., 
are dominant while the more deleterious alleles are recessive, and that each line 
or variety has some unfavorable as well as favorable genes. The hybrid between 
two varieties then has favorable dominants at the maximum number of loci since 
the different varieties will tend to carry different favorable and unfavorable 
genes. 

inbred A X inbred B 
Pi aaBBccDDeeff AAbbCCddEEFF 

F x AaBbCcDdEeFf 

However, the segregation at inbreeding of the F a will restore the homozygous 
recessive condition at one or more of the various loci, and in subsequent genera- 
tions more and more loci will become homozygous recessive and the vigor will 
accordingly decline. It might seem possible to develop a line carrying only favor- 
able dominants in the homozygous condition with vigor as great as that of the F x 
hybrid, but linkage of favorable and unfavorable genes on the same chromosome 
makes this virtually impossible. Even without linkage, if 20 or 30 gene pairs are 
involved in heterosis — probably a low estimate — it would be almost impossible to 
recover such a type from a population of manageable size. 

It should be noted in passing that inbreeding itself is not harmful. 
Cleopatra, the product of generations of inbreeding among the Ptolemies, is 
almost sufficient by herself to confirm this statement. The only effect of inbreed- 



222 • THE MECHANISM OF EVOLUTION 




— ^-^^— ■*— *— ^— ^ l— T^^ 

2.5 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.5 20.5 22.5 24.5 26.5 28.5 30.5 32.5 

Upper limit of class in grams 

PORTER TOMATO 




Upper limit of class in grams 

F, GENERATION 




2.5 4.5 6.5 8.5 10.5 12.5 14.5 16.5 18.5 20.5 22.5 24.5 26.5 28.5 30.5 32.5 
Upper limit of class in grams 

PON DEROSA TOMATO 

Fig. 22-3. Heterosis in tomatoes. Weight per locule in grams in Porter and 
Ponderosa varieties of tomatoes and in their F x hybrid. (Data from Powers.) 



QUANTITATIVE INHERITANCE • 223 

ing is to increase homozygosity. However, since it brings to light otherwise hid- 
den deleterious recessives, the effect is generally harmful. It is highly possible 
that the superstitions, religious taboos, and legal restrictions about incest stem 
originally from its frequently dire biological consequences rather than from the 
more abstruse psychological damage, the latter due to fears that may well have 
developed after the taboos were established. 

The theory of interaction of alleles, later termed overdominance, was 
developed by Fisher and East. The two theories may be compared as shown 
below: 

dominance AA = Aa > aa 

overdominance A\A\ < A\A 2 > A 2 A 2 

In the latter, the heterozygote is superior to both homozygotes. Neither 
A-l nor A 2 is necessarily deleterious, but the heterozygote with two kinds of alleles 
is metabolically superior to either homozygote with only a single allele repre- 
sented. Under this theory, heterosis is directly dependent on heterozygosity; -the 
greater the number of heterozygous loci, the greater the heterosis. With the 
dominance theory, the heterosis is not directly dependent on heterozygosity, for 
it is possible, theoretically at least, for the homozygote to be as vigorous as the 
heterozygote. These two theories are not mutually exclusive, and some evidence 
has been adduced in support of both of them. Furthermore, it should be pointed 
out that a considerable portion of the observed hybrid vigor may be attributable 
to the complementary action of genes at different loci, brought together in favor- 
able combinations by crossing. 

In conclusion, since wild populations of all sorts are generally highly 
heterozygous, it is not surprising to find heterosis as a normal situation in many 
wild populations. Furthermore, quantitative inheritance is of particular im- 
portance in evolutionary studies because crosses between subspecies and species 
typically reveal polygenically controlled differences between them. Evolutionary 
divergence has, therefore, proceeded by means of the gradual accumulation of 
numerous genetic differences. 



► SUMMARY 



Quantitative traits such as size or weight must be meas- 
ured rather than classified, and typically the frequency distribution 
for such a trait in a population takes the form of a normal curve. 
The variability is thus best described in terms of the mean and 
the standard deviation, but does not lend itself to simple Mendel- 
ian analysis. However, the multiple factor hypothesis, which 
postulates a number of genes, each of small effect, has furnished 
a Mendelian explanation for the behavior of quantitative traits in 



224 • THE MECHANISM OF EVOLUTION 

crosses. Hybrid vigor, or heterosis, a special aspect of quantitative 
inheritance, is frequently observed in the hybrid offspring of rela- 
tively inbred parents. The dominance and overdominance theories 
of heterosis explain heterosis as the result of the masking of 
deleterious recessives or of the favorable interaction of alleles, 
respectively. These complementary theories give a genetic expla- 
nation to the heterosis phenomenon. Quantitative traits and heter- 
osis assume particular importance in the study of evolution since 
both have been shown to play a significant role in natural popu- 
lations. 

SUGGESTED READING 

Falconer, D. S., I960. Introduction to quantitative genetics. New York: Ronald. 
Gowen, J. W., ed., 1952. Heterosis. Ames: Iowa State College Press. 
Mather, K., 1949. Biomedical genetics. New York: Dover. 



CHAPTER 



23 



Variation in Natural Populations 



Some of the more fundamental aspects of genetics have 
now been discussed. Our next problem is to relate this informa- 
tion to natural populations and through natural populations to the 
question of the origin and evolution of species. Many students of 
evolution, ecology, paleontology, and taxonomy have long felt 
that the geneticist, cooped up in his laboratory with curtains 
drawn, raising abnormal flies in bottles, or x-raying them to pro- 
duce mutations and chromosomal aberrations, could contribute 
very little to the understanding of phenomena in nature. The as- 
sortment of freaks that the geneticist worked with seemed to have 
little resemblance to the collections of individuals from natural 
populations that these other workers studied. Only recently has 
this viewpoint started to shift, as closer genetic analysis of wild 
populations has begun to reveal the extent of their genetic vari- 
ability. Most of this variability is concealed in the form of hetero- 
zygous recessive genes, but it is, nevertheless, much greater in ex- 
tent than had previously been suspected. 

You would hardly need to be convinced that the human 
species is extremely variable, for people obviously differ in eye 
color, shade of hair, ear size and shape, and so on and on. How- 
ever, you may hesitate before accepting the statement that natural 
populations, whether of mice, lice, or rice, tiger lilies or tigers, 
are also quite variable. Yet, wherever adequate genetic analyses 
have been made, natural populations have been shown to be 
genetically highly variable. Phenotypically, wild populations are 
usually quite uniform, although I have felt it necessary to qualify 



225 



226 • THE MECHANISM OF EVOLUTION 

this last statement ever since I saw, like an apparition, an albino "gray" 
squirrel crossing my yard, and, while trout fishing one day on the North 
Shore of Lake Superior, a purple millefoil growing in the midst of a 
patch of the usual white type, and later, white bluebells growing in the 
same crevice with blue bluebells. These unusual variants, quite clearly, were 
genetic, and a careful survey of a wild population of any species will reveal a 
number of individuals phenotypically distinguishable from the usual "wild type." 
Since Drosophila is so well known genetically, it is not surprising that some of 
the best information of this type is derived from wild Drosophila populations. 
In Drosophila melanogaster, for instance, two percent of several thousand flies 
showed visible differences from the wild type; these affected the size, shape, or 
number of bristles, size, shape, or color of the eyes, wing shape or venation, and 
shape of the legs. On genetic testing, not all were due to mutations, but the 
majority were. 



Genetic Analysis of Natural Populations 

A more thorough analysis of the genetic variability is possible by ex- 
tracting a single chromosome from a wild population and making it homozygous 
in order to reveal its genetic contents (see Fig. 23-1). The general method used 
in Drosophila consists of crossing a single wild male with females from a tester 
stock (for example, A/B) carrying a dominant mutant A to mark one chromo- 
some and a different dominant B to mark its homologue. These dominant genes 
are usually lethal when homozygous. The marked chromosomes carry inversions 
that tend to lead to the elimination of almost all crossovers. A single male show- 
ing A and carrying only one of the two chromosomes from his wild father is 
selected from the F r and crossed again with A/B females. From among the 
progeny of this cross the A males and A females are taken and interbred. In all 
of them the homologue of the A chromosome is identical, descended from a 
single original wild chromosome without crossing over. In the next generation 
the A/A type die while of the remaining flies, % will be expected to be A and 
Y 3 wild type. However, if the chromosome being tested carries a recessive lethal, 
no wild-type flies will appear. Reduced viability or visible effects produced by 
the chromosome are readily detected. In this manner genetic analyses of indi- 
vidual chromosomes from wild populations have been conducted. 

The analysis of a series of second chromosomes from Drosophila in 
New England, Ohio, and Florida showed that 55 percent of these chromosomes 
contained lethal or deleterious recessive genes, most of them at different gene 
loci and hence of independent origin. Many surveys of other species of Dro- 
sophila have produced similar results, and one such study led to the conclusion 
that in less than 3 percent of the flies studied was there no harmful mutation in 
either the second or third chromosomes. When the other three pairs of chromo- 



VARIATION IN NATURAL POPULATIONS • 227 

A 




Fig. 23-1. Generalized method of genetic analysis of individual chromosomes 

from wild populations. 



somes are taken into account, it is clear that there are practically no individuals 
who do not carry at least one deleterious recessive mutant gene. Since these 
deleterious genes are balanced by their dominant wild-type alleles, a given pair 
of chromosomes may carry several harmful genes that are not expressed. If the 
mutants are closely linked lethals, a balanced lethal system will be established, 



+ 



+ 3 1 4 



li +: 



+ ■ 



in which only the heterozygotes survive. Unless there is some means of detecting 
the homozygous lethal zygotes, such a balanced lethal system will appear to be a 
true-breeding homozygous strain. 

Mutations are constantly recurring in both wild and laboratory popula- 
tions, which replenish the lethals and the deleterious mutants that are being 



228 • THE MECHANISM OF EVOLUTION 

eliminated from the population by natural selection against the homozygotes. 
However, the harmful effects of these mutants are apparently not confined to the 
homozygotes, for a study of the viability of individuals heterozygous for lethals 
showed them on the average to be 4 percent less viable than the homozygous 
wild-type individuals. Thus, the damage wrought by deleterious genes due to 
their insidious effects on heterozygotes over a number of generations may be 
greater than the single genetic death of the homozygote. 

One further point to be noted and perhaps emphasized is that the muta- 
tions revealed by the genetic analyses of wild populations of Drosophila were no 
different in kind from those studied by the geneticist in the laboratory for many 
years. Furthermore, Drosophila are not unique in carrying large numbers of con- 
cealed recessives; they are observed frequently in other species as well. The most 
striking variant I ever saw was an albino snapping turtle, but adequate sampling 
of any species will reveal some individuals distinctly different from the so-called 
"wild type." More careful study will show that the extreme types grade into less 
extreme types and on into quantitative differences so that the variability is in 
degree rather than in kind. 

The phenotypic variation in wild populations has frequently been as- 
cribed to environmental effects, and without doubt this is often true. A com- 
parison of the growth of a field of corn during a wet summer and a dry one will 
reveal how great an influence the environment can have. Hence, there has been a 
general tendency to regard all of the differences exhibited between populations 
of a species living in different habitats to be nongenetic. However, when repre- 
sentatives from different populations are grown together under the same environ- 
mental conditions, many of the differences remain. For an example, let us con- 
sider a cinquefoil, Potentilla glandulosa, which grows in California. As you go 
inland from the Pacific, this plant is found in a variety of habitats: the Coast 
Range, with low elevation and a mild climate; the foothills of the Sierra Nevada 
with both dry slopes and open meadows and a continental climate of hot sum- 
mers, cold snowy winters, and rainy springs; subalpine and alpine habitats up in 
the Sierras with a short growing season, cold winters, and abundant precipita- 
tion. Reciprocal transplants of individuals from each of these habitats to all of 
the others showed that the differences between them were hereditary. The popu- 
lations had become genetically adapted to their own particular habitats, and 
hence, even though not far removed geographically, they belonged to different 
races or ecotypes. Furthermore, even though all were members of the same 
species, none of the lowland races could even survive in the alpine environment. 
See Fig. 23-2. 

When the different races were crossed, no two individuals among some 
1600 F 2 progeny were alike, and the minimum number of genes differentiating 
these races was estimated to be from 60 to 100. Such a burst of recombination 
indicates clearly that the genes in one race differ from their alleles in other races, 



VARIATION IN NATURAL POPULATIONS • 229 

and that the observed differences are not due merely to the direct effects of the 
different environmental forces operating on similar genotypes. 

Chromosomal Variation 

In addition to the genie variability existing within populations and be- 
tween populations of the same species, chromosomal rearrangements are found 
frequently and in some cases regularly in wild populations. The most detailed 
study of inversions in nature has been made in the genus Drosophila with the 
inversions in the third chromosome of D. pseudoobscura. Many inversions, 
which rearrange the banded structure in the salivary chromosomes, have been 
identified. The different arrangements can be related to each other by the fact 
that one pattern can give rise to another by a single inversion. For example, 

Pattern 1 ABCDEFGHIJ 

T T 

break break 

Pattern 2 ABCGFEDHIJ 

T T 

break break 
Pattern 3 ABCGEFDHIJ 

These three patterns are clearly related to each other, though 3 and 1 are not 
directly related but only through 2. Three sequences for their origin are possible: 

1 > 2 > 3 

1 < 2 < 3 

1 < 2 > 3 

In this manner, a phylogeny of these inversion types has been con- 
structed, including more than 20 different inversions and two other species as 
well, D. miranda and D. persimilis. 

Translocations are also found in natural populations, the best-studied 
case being the Jimson weed {Datura stramonium) . Datura has 12 pairs of chro- 
mosomes, but crosses of different races give rings of 4 or 6 chromosomes rather 
than 12 bivalents. The cause of these rings, as we have seen, is the synapsis of 
chromosomes with translocations. At least 7 translocations have been identified 
from different races of Jimson weed, and translocations have been observed in 
many other species of plants and animals. The evening primrose is the most 
spectacular case, having translocations as a regular part of the genetic mechanism 
of individuals in the same population. 

Natural polyploids are especially common among plants, for most 
genera of plants have polyploid members. In the genus Solatium (nightshade, 
potato, eggplant, and so on) the following numbers have been identified: 



230 • THE MECHANISM OF EVOLUTION 






% «fev 













:; 



F##. 23-2. Representatives of four 
subspecies or ecological races of the 
cinquefoil, Potent ilia glandulosa, 
grown in a uniform garden at Stan- 
ford. The different races come from 
central California along a 200 mile 
transect from the coast inland into 
the Sierra Nevada. Races shown 
from west to east are: bottom row, 
typica; second row, reflexa; third 
row, hanseni; top row, nevadensis. 
All to the same scale. (Courtesy of 
Clausen and Heisey.) 

In — 24, 36, 48, 60, 72, 96, and 120. Polyploids are also known in such diverse 
groups as strawberries, grasses, lilies, spiderworts, cotton, tobacco, iris, mints, 
willows, and sunflowers. In these cases, the polyploids are higher multiples of 
some basic haploid number. In some cases, the postulated ancestry of an apparent 
allopolyploid has been confirmed by the experimental resynthesis of the poly- 




VARIATION IN NATURAL POPULATIONS • 231 



*€%., 



y |||| 



\ M r HI 




\SvV>. 



V .-..*. *<M-Y*«5 




*r-..'. 









^ g* 



yRmBBm 



M% 




^fX^^?' -£ 



. /* 







ploid from the diploid ancestors. One such case is the synthesis of the allo- 
polyploid Galeopsis tetrahit from the diploids, G. pubescens and G. speciosa 
(Fig. 23-3). 

This brief survey should make it clear that the genie and chromosomal 
changes found during observation and experiment in the laboratory and in ex- 



232 • THE MECHANISM OF EVOLUTION 




Fig. 23-3. The first successful resynthesis of a naturally occurring species, 
Galeopsis tetrahit. Shown are the ancestral diploid species, G. speciosa 
(left) and G. pubescens (right), and the artificial tetraploid (center) derived 
from them, which is indistinguishable from wild G. tetrahit. (Courtesy of 

Miintzing.) 



perimental plots have their counterparts in wild populations. There is no intrinsic 
difference between the variations seen in the laboratory and in the field. Their 
nature and their causes are the same, and the study of evolution can safely be 
based on the knowledge about heredity and variation gained by experimentation. 



SUMMARY <- 



By means of special techniques, natural populations that 
usually appear quite uniform can be shown to carry a sizable store 
of genetic variability in the heterozygous condition. Although 
much of the variation between individuals and between popula- 
tions may be environmental, the evidence is clear that in most 
cases there is a genetic component as well, especially when the 
populations are living under different ecological conditions. In 
addition to genie differences, chromosomal variation frequently 
forms a characteristic part of the hereditary variability of a species. 
Thus, for example, in many species translocation or inversion 
heterozygotes are routinely found, and many plants are clearly 
polyploid in origin. 



VARIATION IN NATURAL POPULATIONS • 233 

SUGGESTED READING 

Darwin, C, 1872. The origin of species. New York: Mentor Books (1958). 

Dobzhansky, Th., 1951. Genetics and the origin of species, 3d ed. New York: 
Columbia University Press. 

Mayr, E., 1942. Systematic s and the origin of species. New York: Columbia Univer- 
sity Press. 

Stebbins, G. L., 1950. Variation and evolution in plants. New York: Columbia 
University Press. 







cL&* 



CHAPTER 24 rtUtidf-tffy 'f^^fo 

chapter ^ ^ c ^^j^ f J~ '^ryrt t^- 
Genetics of Populations 






Evolution has been termed "descent with modification," 
by Darwin. Further consideration is needed to clarify this concept. 
The first question to answer is "What is it that evolves?" It is not 
the individual, for the individual lives and dies with a fixed geno- 
type that does not change; rather, the species is the evolving unit. 
Even without a formal definition of a species, it is nevertheless 
clear that a species consists of a number of individuals; it is a 
population, and evolution is a population phenomenon. For evo- 
lutionary change to occur, a population with one set of hereditary 
characteristics must in some way give rise to a population with a 
different set of hereditary characteristics. Since inherited traits are 
controlled by the genes, evolution can be redefined as a change in 
the kinds or frequencies of genes in populations. The problem 
then becomes to discover how the frequency of a gene already 
existing in the population may change, or how new types of 
genes, originating by mutation, become incorporated into the 
population. In order to study the genetics of a population, it is 
necessary to consider it, not as a group of individuals, but rather 
as a pool of genes from which individuals draw their genotypes 
and to which they in turn contribute their genes to form the pool 
for the next generation. 

Up to this point we have been concerned with gene 
effects in individuals and with the results of controlled matings 
between individuals of specified genotypes. Most knowledge and 
prediction in genetics is based on this type of experimentation. 
The problem now, however, is to consider the operation of 
heredity in a natural variable population of freely interbreeding 

234 



GENETICS OF POPULATIONS -235 

individuals. How are the genes present in the members of such a Mendel- 
ian population transmitted and distributed to succeeding generations? In 
order to understand an extremely complex situation, it is best to study it in its 
simplest possible terms. By restricting our attention to the bare essentials of events 
at a single gene locus, we can discover the underlying principles. Once estab- 
lished, there is no reason to suppose that these basic principles do not hold in 
the more complex as well as in the simple cases. 



The Hardy-Weinberg Equilibrium 

Let us consider first what happens in a population in which selection, 
mutation, and other evolutionary forces are not operating. In man, the ability 
to taste phenylthiocarbamide (PTC) is inherited as a simple dominant. Tasters 
of PTC are, then, of two genotypes, TT or Tt; nontasters are homozygous reces- 
sive, tt. Since very few persons are aware of either their genotype or phenotype, 
marriages occur at random with respect to this trait. People do not ask their 
potential mates whether they like PTC, for they simply do not care. There is, 
therefore, neither preference nor avoidance of a mate because of his PTC sensi- 
tivities, and mating on this score is said to be at random. 

There is no simple answer to the question of how frequent tasters and 
nontasters should be in a human population. There will be no classical 3:1 
Mendelian ratio, nor will the dominant tasters necessarily be more frequent than 
the recessive nontasters, for there is no known selective advantage of one type 
over the other. A population may contain any proportion of tasters and non- 
tasters, depending on the frequencies of the dominant and recessive genes. In a 
population of 100 people there will be, since they are diploid, 200 genes at the 
taster locus. Let us suppose that there are 20 TT, 40 Tt, and 40 tt. The frequency 

, . 40+40 40+80 

of gene J is p = — — — == .4. The frequency of gene t is q = — tt: - = -6. 



200 



20+60 



= .4 and a — 



200 
60+60 



If there are 10 TT, 60 Tt, and 50 tt, p = , 

' r 200 l 200 

Hence, even though the distribution of these genes in individual genotypes is 
different, their frequencies in the two populations are identical. If mating is at 
random in the former population, the different types of matings will occur in 
proportion to the frequency of the various genotypes as shown below. 



9\ 


TT 

.2 


Tt 

A 


tt 
.4 


7T .2 


TT = .04 


TT = .04 

Tt = .04 


Tt = .08 


Tt .4 


TT= .04 
Tt = .04 


TT= .04 

Tt = .08 
tt = .04 


Tt = .08 

tt = .08 


tt .4 


Tt = .08 


Tt = .08 
tt = .08 


tt= .16 



Summing up, we find TT = 
Tt = 



and still p 
and q 



tt = 
24 = 
24 = 



.16 

.48 

.36 

.4 

.6 



236 • THE MECHANISM OF EVOLUTION 



But this method is too cumbersome. If mating is truly random, then the combi- 
nation of gametes is at random, and it is possible to deal directly with gene fre- 
quencies in the gametes to obtain the same result. 



9 \ 


p=im 

.4 


f-XO 

.6 


P -4 


^=.16 


pq= .24 


t .6 


pq = .24 


q 2 = .36 



f + lpq+ q 2 =l 
.16+ .48 + .36=1 

(TT) (7>) O) 



Furthermore, it should now be clear that even the checkerboard is unnecessary, 
for the relation between gene frequency and genotype frequency can be expressed 
as the binominal (p + q) 2 = 1. From the binominal expansion, it is clear that 
in_.a large random mating population not only the gene frequencies but also the 
genotype frequencies will remain constant. In a random mating population with 
p — A and q = .6, the equilibrium frequencies will be TT ~ 16 percent, Tt = 
48 percent, and tt = 36 percent. 

It should be noted that if only the frequency of the homozygous reces- 
sive class is known, the frequency of the recessive gene can be calculated. For 
example, if 9 percent of a human population has red hair, then 

f 



far) = ? 2 


= 0.09 


Also, if D = /(RR) 


fa) =1 


= V^09 = 0.3 


H = /(Rr) 


fOO =p 


= 1 - n = 0.7 


R = far) 


fQRK) = p* 

/(Rr) = in 


= 0.49 

- 2(.3) (.7) = 0.42 


Then p = D + H+R 

j R+ W 
and ?= D + H+R 



Thus can the entire population be described. Perhaps the most surprising fact to 
emerge is that 42 percent of a random mating population must be heterozygous 
carriers of the recessive gene that is expressed homozygously in only 9 percent of 
the population. This disparity becomes even greater for the less frequent reces- 
sives. For instance, if q 2 = 0.01, 2pq = 0.18; if q 2 = 0.0001, 2pq = 0.0198. 
This equilibrium is known as the Hardy-Weinberg equilibrium, after 
the men who independently derived the equation and understood its implica- 
tions. To state the law more explicitly, in a large, randomly mating population, 
in the absence of mutation and selection, - the relative frequencies of the genes 
will tend to remain constant from generation to generation. Darwin, because of 
his belief in blending inheritance, thought that variability decreased each genera- 
tion and had to be constantly replenished. However, from the Hardy-Weinberg 
equation, it is clear that so long as TT, Tt, and // survive and reproduce equally, 



GENETICS OF POPULATIONS • 237 

the variability in the population will be unchanged, and the equilibrium then is 
a conservative factor in evolution. In fact, evolution can now be redefined quite 
simply as a shift in the Hardy -Weinberg equilibrium. The factors responsible for 
bringing about such shifts are mutation, natural selection, migration or gene 
flow, and random genetic drift, each of which we shall consider in greater detail 
here and in following chapters. 



Mutation 

Let us first examine the effects of mutation on gene frequencies. Sup- 
pose that T mutates to / at the rate of 1 in 10,000 gametes per generation. Muta- 
tion can then be said to be causing an increase in the frequency of /, for the 
proportions of T and t are changing. J[n due time, if no other force intervenes, 
no T genes would be left at all, and the entire population would be //. Such a 
change would be very slow and very unlikely, but theoretically mutation pressure 
alone could bring about evolution, in this case eliminating the taster gene. 

However, reverse mutations also can occur, usually at different rates. 
Suppose that / mutates to T at the rate of 5 per 100,000. 

Let u = T -> t = 0.00010 

v = t -> T = 0.00005 

Then the change in frequency of T (Ap) will equal the net change brought 
about by these opposed mutation rates. 

increase in T = vq 
decrease in T = up 



Ap = vq — up 

Since the reverse mutations are occurring, the population can never be- 
come homozygous for one type of allele. Hence, an equilibrium will be estab- 
lished at the point where the number of mutations from T— >/ just equals the 
number of mutations from /->T; in other words, when Ap = vq — up — 0. 
This equation can then be transformed as follows : 

vq = up 
v(l -p) = up 

v — vp = up 
up + vp = V 
p(u + v) = V 



4i 



A _ vt| 
P ~ U+ V 



238 • THE MECHANISM OF EVOLUTION 



it should be noted that the equilibrium value of p is dependent only on 
the mutation rates and is independent of the initial gene frequencies, which may 
range anywhere then from p = to p .== 1. For the rates given above, 



a = 0.00005 

V 0.00010 + 0.00005 ~ °' 333 

q = 0.667 



T ^t l he 5, wi11 - be twice as man y ^cessive / genes mutating half as often as the 
dominant' f genes, and the result is an equilibrium since the absolute numbers 
of mutations are equal. 

Even though evolutionary change due to the action of mutation pressure 
is theoretically possible, the course of evolution is not controlled to any great 
extent by mutation. Mutation is a limiting factor rather than a controlling factor 
in evolution. 



SUMMARY < 



The frequency of a gene may be denned as the propor- 
tion that a given allele forms of the total of all the different 
kinds of alleles at this locus in the population. Random mating 
occurs when any male in a population has an equal chance of 
mating with any female. Hardy and, Weinberg showed that in a 
large, randomly mating population, in the absence of mutation 
and selection, the gene frequencies will remain constant, and the 
-I5"i tic var i a biiity thus is conserved. However, if mutations occur, 
mutation pressure will tend to cause shifts in gene frequency! 
Where reverse mutations also occur, a new equilibrium will be 
established that is solely determined by the mutation rates. 

SUGGESTED READING 

Cold Spring Harbor Symp. Quant. Biol., Vol. 20, 1955. "Population genetics." Long 
Island Biological Assn., New York. 

Haldane, J. B. S., 1932. The. causes of evolution. New York: Harper. 

Lerner, I. M., 1950. Population genetics and animal improvement. New York: Cam- 
bridge University Press. 

Li, C. C, 1955. Population genetics. Chicago: University of Chicago Press. 



CHAPTER 



25 



Natural Selection 



The primary factor controlling the course of evolution is 
natural selection. We have already discussed the Darwinian con- 
cept of natural selection, which assumed a population more or less 
stable numerically with a reproductive rate far higher than neces- 
sary to ensure the maintenance of the population's size. Because 
the population is variable, the ensuing deaths occur more fre- 
quently among the less well-adapted individuals, and the better 
adapted types survive. Darwin placed emphasis on predation and 
on competition, and to many, natural selection came to signify a 
concept of nature, red in tooth and claw. Another aspect of 
Darwinism, neglected in recent years, was his concept of sexual 
selection due either to male competition or female preference. 

The modern concept of natural selection involves a subtle 
change in emphasis from differential survival to differential repro- 
duction. From the standpoint of evolution, it matters little 
whether an individual survives to the age of 2 or to 102; if he 
dies without offspring, his genes are lost from the population. 
Any and all factors that bring about differential reproduction — 
the production of more progeny by one hereditary type in propor- 
tion to its numbers than by the other types — are factors in 
natural selection. Included among these factors are survival and 
longevity, fertility and fecundity, competition and cooperation, 
disease and parasite resistance, food requirements, physiological 
tolerances, sexual selection, color patterns, behavior patterns, and 
so on and on. To the extent that any of these factors, trivial or 
major, affects reproductive fitness, they have adaptive value; and 



239 



240 • THE MECHANISM OF EVOLUTION 

to the extent that the differences are controlled by genes, the favorable 
genes will increase in frequency while the less favorable genes will decline 
in frequency each generation. The net effect is the production of organ- 
isms well adapted to survive in their particular environments. Since many, 
many selective pressures operate, it is clear that the organism must make 
some adjustment to all of them. Hence, the final phenotypes are compromises 
that permit the organism to make the best possible adjustment to all the various 
selection pressures, but no one adaptation is apt to be perfect. Natural selection, 
then, brings about adaptation; it may be to a changing environment, or it may 
be an improvement in the existing adaptations to a fairly stable environment. 
Evolution may thus be thought of also as successive or perhaps in some cases 
progressive adaptation. 

A great deal has been written about the theory of natural selection. It 
has been hailed as a monumental advance, but it has also been severely criticized 
and even regarded as completely erroneous. We cannot hope to pursue all of the 
avenues open to discussion, but we can point out that the basis of many of the 
objections seems to be the difficulty in visualizing how such enormously complex 
systems as the human eye, the electric organ in fishes, the insect societies, and 
the adaptively appropriate patterns of instinctive behavior could have arisen as 
the result of gradual changes emanating from such an apparently simple process 
as differential reproduction. The fault, however, lies more with the imagination 
than with the process of natural selection, for selection almost inevitably tends 
toward the improvement of adaptation, and these examples represent some sort 
of adaptive pinnacle. Although a detailed history of the origin of many of the 
more bizarre adaptations is not yet possible, it is by no means impossible that this 
history may eventually be learned. 

That natural selection gave rise to a brutal concept of nature made the 
theory of natural selection distasteful or even unacceptable to many people. The 
idea of competition or the struggle for existence was regarded as a threat to any 
higher concept of man or of nature. Distasteful or not, predation, competition, 
and parasitism are biological facts of life. Anyone who has spent any time in the 
field realizes that death is a very casual, commonplace affair among living things. 
Predators live at the expense of their prey; parasites, though less demanding, at 
the expense of their hosts. Members of the same species may compete for food, 
space, light, or other essentials. In fact, intraspecific competition may be even 
more severe than the competition between different species. In a crowded group 
of seedlings only a few will survive the competition for light and space. This 
contest is bloodless but fatal nonetheless to the losers. Similarly, under crowded 
conditions the growth of small tadpoles is inhibited by the presence of larger 
tadpoles of the same species, and they eventually die despite the presence of 
abundant food. We may be repelled by the garter snake that engulfs a living 
leopard frog inch by inch, or by the leech that drains its blood, leaving it in a 



NATURAL SELECTION • 24 1 

moribund condition, but this is their normal way of life. Thus natural selection 
does involve a struggle for existence, and attempts to gloss over this fact do an 
injustice to the concept. 

On the other hand, to regard selection as nothing more than a bitter 
struggle to survive is just as erroneous, for biological success depends on many 
factors in addition to escaping death. Cooperative behavior may also contribute 
to reproductive fitness, and may increase as the result of natural selection. Care 
of the young in birds and mammals, division of labor in colonial species such as 
protozoans, coelenterates, and insects, and the complex group behavior of fishes, 
birds, and mammals have all arisen during the course of evolution. In most cases 
they clearly are adaptive and contribute directly or indirectly to reproductive 
fitness, and therefore must have been favored by and developed under the influ- 
ence of natural selection. Thus, natural selection must be regarded as being 
responsible not only for the unending struggle for existence but also for many 
of the forms of altruistic behavior. In some of these cases, the behavior has dire 
consequences for the individual — for example, the bee, which dies once it has 
stung an invader — but if the chances of survival of the colony are thereby im- 
proved, this behavior will be favored by selection. 

Natural selection in itself does not admit of being judged as good or 
evil. We may regard its consequences as either good or bad, but they flow from 
the sole criterion in selection, reproductive fitness. Those factors, whatever their 
nature, that increase fitness will tend to be favored by natural selection; those 
decreasing it will tend to be eliminated. 

Artificial Selection 

Since there are sometimes questions or doubts as to the efficacy of selec- 
tion, it may be well to consider some examples of the operation of selection. A 
magnified, if somewhat distorted, view of evolution is obtained from an exami- 
nation of the results obtained by artificial selection. The changes wrought by man 
in developing new breeds are, strictly speaking, evolutionary changes, since a 
population with a new set of hereditary traits is derived from an ancestral popu- 
lation; but they are on a small scale and are directed toward man's benefit or 
amusement rather than that of the species. Certainly no dachshund or Pekingese 
would be likely to consider himself especially well equipped to make a go of it 
on his own. A well-documented history of the development of a new breed of 
animals is that of the Santa Gertrudis cattle on the fabulous King Ranch in 
Texas. The ranch is in southern Texas where ordinary beef cattle — such breeds 
as Shorthorn, Aberdeen Angus, and Hereford — did not thrive in the semi- 
tropical rather arid climate, for they were bothered by the heat and ticks and did 
not grow well on the available grasses. The Brahma cattle of India thrived in 
this climate, but were of poor quality. Crosses and back-crosses of Shorthorn and 



242 • THE MECHANISM OF EVOLUTION 

Brahma, accompanied by selection for the desired beef qualities and ability to 
withstand the climate, ultimately produced a population with approximately 
7 / s of its gene pool derived from the Shorthorns and l/g from the Brahmas (see 
Fig. 25-1). This new breed is heat and tick resistant and gains better on grass 
feeding than any other breed. A couple of footnotes may be added to this story. 
Dissatisfied with the type of grass on their range, the owners of the King Ranch 
developed new varieties of grass and reseeded vast areas of the ranch with the 
improved type. Furthermore, their success in selecting and breeding horses for 
their ability to run faster than other horses has paid off at the Kentucky Derby 
and elsewhere. The success of breeders in all instances is due basically to chang- 
ing the frequencies or types of genes and gene combinations in the population 
of animals or plants with which they are working. These changes, secured by 
artificial selection, are brought about by the differential reproduction of the 
favored types. 

Selection for Resistance 

The Santa Gertrudis cattle have been developed within the past 50 
years, and many other evolutionary changes in this interval can be cited. The 
introduction of chemotherapeutic agents and antibiotics was followed by the 
origin of strains of bacteria that were resistant to these agents; for instance, 
strains resistant to the various sulfas, terramycin, aureomycin, penicillin, and 
streptomycin are known. Moreover, strains of bacteria actually dependent on 
streptomycin for normal growth have been discovered. These changes are the 
result of the drug having killed all of the microorganisms except those carrying 
mutations to resistance, which then become progenitors of the resistant strains. 
The mutations have been shown to be random and not produced as a specific 
result of treatment by the antibiotic, for by suitable techniques, mutations to 
resistance have been isolated in bacteria never exposed to the antibiotic at all. 
These facts lead to caution in hailing any new wonder drug as the final solution 
for any particular disease, for the possibility always exists that the disease organ- 
ism will mutate to resistance. Furthermore, the indiscriminate use of any anti- 
biotic is inadvisable simply because it will increase the frequency of the resistant 
mutants in the bacterial population and make the disease more difficult to control 
if most infections are due to resistant rather than susceptible organisms. Therapy 
has been directed toward using combinations of drugs, since the chances of in- 
dependent mutations to resistance to two or more antibiotics in a single bacterial 
cell are vanishingly slight. 

Hydrogen cyanide is commonly thought of as one of the deadliest 
poisons, yet resistant strains of the scale insects attacking citrus fruits have 
evolved. Similarly, the widespread use of DDT caused in insect populations a 
selection pressure that led to the development of resistant strains of mosquitos, 




Fig. 25-1. The genesis of a new breed of beef cattle. Hybridization between 

Brahmas (above), Shorthorns (center) followed by selection produced the Santa 

Gertrudis breed (below). (Courtesy of Snyder and David.) 



244 • THE MECHANISM OF EVOLUTION 

house flies, and body lice. They have appeared in many different parts of the 
world, often within two or three years of the introduction of DDT. 

Bacteriophages are viruses that attack and destroy bacteria. Bacteria that 
are resistant to phage can arise by mutation, but the virus can also mutate to 
forms able to attack the previously resistant bacteria. A similar situation exists in 
wheat-stem rust. As plant breeders develop new varieties of wheat that are re- 
sistant to the currently prevalent strains of rust, new mutant strains able to attack 
the resistant wheat increase sharply in frequency until a new outbreak of stem 
rust occurs. The plant breeder must try to keep one jump ahead, but as things 
stand, he is not likely to work himself out of a job. These situations involving 
two different species are more complex because both host and pathogen (the 
disease-causing agent) are capable of evolution, and each exerts a selective pres- 
sure on the other. 



The Baldwin Effect 

A great deal still remains to be learned about the ways in which natural 
selection operates to bring about adaptation, for it is a subtle as well as a power- 
ful force. Furthermore, the appeal of Lamarckianism has persisted because it has 
seemed that many of the more remarkable adaptations could have arisen only in 
direct response to the environment or to the needs of the organism rather than 
by the operation of natural selection on random mutations. Some recent experi- 
ments by Waddington on what is known as the Baldwin effect have been most 
revealing. A number of wild-type fruit flies were subjected to temperature shock 
during development. As a result of this treatment some of these flies were cross- 
veinless. The crossveinless condition of the wings was not due to mutations in- 
duced by the heat treatment, however, for untreated progeny of these flies were 
wild type and could be shown not to carry a crossveinless mutation. Such an 
environmentally induced condition that simulates the phenotype of a genetic 
mutant is known as a phenocopy. Nevertheless, the crossveinless flies were bred 
together, the offspring given heat shock during development, and the crossvein- 
less offspring again selected and interbred over a period of several generations. 
After about 15 generations of selection, the heat treatment was discontinued, but 
crossveinless flies still continued to appear in these stocks. 

At first thought, this result seems clearly to indicate Lamarckian in- 
heritance of acquired characteristics. Actually it does not, but it may serve to 
reconcile to some extent Lamarckianism with the theory of natural selection. In 
the first place, the initial wild- type stock had not been selected or inbred and was 
therefore undoubtedly heterozygous. Among this array of genotypes were some 
that could produce the crossveinless phenotype, but only under the unusual en- 
vironmental conditions provided by the temperature shock. When these genes 
were brought to expression, selection then became possible. Experiments with the 



NATURAL SELECTION • 245 

crossveinless stock resulting from selection showed that the crossveinless condi- 
tion was controlled by polygenes or multiple factors rather than by a single gene 
locus. Therefore, selection over a number of generations had simply increased 
the frequency of these genes in the population to the point where individual 
genotypes carried enough of them to cause the crossveinless phenorype even in 
the absence of temperature shock. In other words, it could be said that selection 
had lowered the threshold for crossveinless. It should be noted that even the 
ability to produce the so-called phenocopies was not independent of the geno- 
type. In these experiments, a mechanism has been revealed by which the re- 
sponses of individuals to new environmental pressures have been incorporated 
through natural selection into the population as a whole. Thus could the transi- 
tion from individual physiological adaptation to population genetic adaptation 
be made. The distinction between these two types of adaptation is obviously not 
clear-cut, because, just as the adaptation of a population to its environment is 
determined by its genetic composition, the adaptive responses possible to an indi- 
vidual are also controlled by his genotype. Therefore, even though many adaptive 
changes may appear Lamarckian, they may nevertheless have a completely reason- 
able explanation under the theory of natural selection. 



The Theory of Selection 

With these examples in mind, let us now consider the way in which 
gene frequencies change because of selection. The theory of selection is very 
simple. Suppose that A and a alleles are present in a population with equal fre- 
quency, but that only 99 a genes are transmitted to the next generation for every 
100/1. The recessive a gene is therefore at a slight selective disadvantage to the 
dominant. The selection coefficient, j", is a measure of this disadvantage and is 
obtained as follows: 

1 - s 99 



1 100 

s = 0.01 

Most selection pressures operate on the diploid or zygote phase rather 
than on the haploid or gametic stage. A common type of zygotic selection is that 
against deleterious recessive homozygotes with the homozygous dominants and 
the heterozygotes equally viable. For this situation the change in frequency of 
the dominant A gene is calculated as follows : 



genotype 


AA 


Aa 


aa 


total 


r requency before selection 


f 


ipq 


4 2 


1 


frequency after selection 


f 


2pq 


q\l - ,3 


1 - sq 2 



246 • THE MECHANISM OF EVOLUTION 

Here, s measures the selective disadvantage of the aa type. 

Ap = pi — p p = /(/4) in generation 

p 2 + pq pi == f(A) in generation 1 



Pl= 1- sf 



z t±n _ t= m 2 

1 - sq 2 F 1 - sq' 



If sq 2 is small, the denominator is essentially equal to 1, and further simplifica- 
tion is possible to 

Ap = spa 2 

If s, p, or q is small, selection will act only very slowly. Therefore, selection pres- 
sures are most effective at intermediate gene frequencies. From the equation it is 
clear that selection will have no effect at all if s, p, or q equals zero. In other 
words, one allele must have a selective advantage and both alleles must be 
present in the population for selection to operate. Hence, selection is ineffective 
in a homozygous population, no matter how great the environmental variation 
may be. As early as 1910, Johannsen showed experimentally the futility of selec- 
tion on environmental variation. As a result, Darwin's ideas on selection have 
been modified and clarified, for he did not make a clear distinction between 
hereditary and environmental variation and believed natural selection could act 
on both. He was inclined to accept Lamarckian inheritance of acquired charac- 
ters, though at times he also seemed to have some reservations about the possi- 
bility that environmentally induced changes could become hereditary. 

If selection is directed against a deleterious dominant, the gene is ex- 
pressed and exposed to selection in both AA and Aa individuals. If no dominant 
individual leaves progeny, the gene will be eliminated except for new mutations, 
in a generation. Even if selection is not complete, it is still very effective, for all 
of the dominant genes are exposed to selection. It is for this reason that deleteri- 
ous dominant mutations are so rarely observed in wild populations, and a fair 
proportion of those seen arise from new mutations. 

On the. other hand,. selection against a harmful recessive gene is consid- 
erably less effective. The gene is carried by both Aa and aa, but the full force of 
selection acts only on the aa individuals. Since the defective homozygotes aa are 
normally less frequent than the heterozygotes Aa, the frequencies being as q 2 
{aa) is to 2pq (Aa), a large proportion of the deleterious recessives are not ex- 
posed to selection. Furthermore, the less frequent a becomes, the greater the 
proportion of the recessives carried by the heterozygotes, and hence the less 
effective selection becomes. Even-recessive lethals may be present in a fairly high 
frequency, for when no recessive homozygotes survive or reproduce, affected 
' \ individuals will continue to appear as the offspring of heterozygous normal 
parents. 



NATURAL SELECTION • 247 

Selection and Mutation 

If selecti on against^ an unfavorable recessive were to continue over a 
long period of time, eventually the recessive might be expected to_be eliminated 
entirely from the population. However, recurrent mutation will periodically add 
additional recessives to the population before the recessive is completely gone. 
The forces of selection pressure and mutation pressure will therefore tend to be 
opposed under these circumstances, and an equilibrium between these opposing 
forces will be established. Since 

Ap = spq 2 — up 

where spq 2 is the effect of zygotic selection against the homozygous recessive aa 
and u is the mutation rate from A to a, then at equilibrium 

Ap = spq 2 - up = 
spq 2 = up 

and a 2 — - 

s 

Thus the frequency of appearance of the homozygous recessive type aa (q 2 ) is 
determined by the relationship between the mutation rate and the selection co- 
efficient. In the case of a recessive lethal s equals 1, and q 2 = u directly. For 
example, if one person in 40,000 dies owing to a homozygous recessive lethal 
condition, the mutation rate to the recessive also equals 1/40,000. Moreover, q = 
1/200, p = 199/200, and 2pq, the frequency of the heterozygotes (Aa), equals 
398/40,000, or approximately 1 percent. Thus even though the gene is lethal, 
less than 1 percent of these lethal genes are exposed to selection each generation, 
and their frequency in the population may remain surprisingly high. 

Evolutionary change comes about, then, as a result of the joint effects of 
mutation and natural selection. New kinds of genes originate in a population by 
mutation and may increase in frequency because of either recurrent mutation or 
chance events, for selection is relatively ineffective at extremely low gene fre- 
quencies. As gene frequencies increase, selection becomes increasingly important 
in determining the ultimate fate of the genes in the population. Wi thoutj ihe 
genetic variability originally supplied by mutation, natural selection is powerless 
Jx)j3perate. Without the sifting and winnowing of natural selection, mutation 
pressures would soon reduce a population to an array of freaks. 



248 • THE MECHANISM OF EVOLUTION 

SUMMARY < 



The essence of natural selection is differential reproduc- 
tion. Thus, many factors in addition to survival may be 
significant. Natural selection is the mechanism through which 
adaptation is achieved, for the better adapted individuals leave 
proportionately more offspring. The concept of natural selection 
as a "struggle for existence" or "the survival of the fittest," 
though correct in many cases, is incomplete, since cooperative be- 
havior or even altruism may also be developed by natural selection 
if they contribute to reproductive fitness. The efficacy of selection 
can be demonstrated in domesticated species as well as in natural 
populations. Perhaps the most unusual example was the work on 
the Baldwin effect, which demonstrated that an apparently 
Lamarckian change could be explained within the existing theoret- 
ical framework. Selection can be effective only in heterozygous 
populations, and is thus without effect on environmental varia- 
tion. Selection against dominant genes will be considerably more 
successful than against recessives, since the recessives in the heter- 
ozygous condition are not exposed to selection. Ordinarily, selec- 
tion pressures and mutation pressures are opposed, and an equi- 
librium between the origin of new genes through mutation and 
their elimination by selection is achieved. 

SUGGESTED READING 

Darwin, C, 1872. The origin of species. New York: Mentor Books (1958). 
Dobzhansky, Th., 1951. Genetics and the origin of species, 3d ed. New York: 

Columbia University Press. 
Fisher, R. A., 1930. The genetical theory of natural selection. Oxford: Clarendon 

Press. (Also Dover, New York.) 
Lerner, I. M., 1958. The genetic basis of selection. New York: Wiley. 
, 1959. "The concept of natural selection: a centennial view," Proc. Amer. 

Philosophical Society, 103(2) :173-182. 
Muller, H. J., 1949. "The Darwinian and modern conceptions of natural selection," 

Proc. Amer. Philosophical Society, 93(6) :459-470. 
Schmalhausen, I. I., 1949. Factors of evolution. The theory of stabilizing selection. 

(I. Dordick, tr.). Philadelphia: Blakiston. 
Sheppard, P. M., 1958. Natural selection and heredity. London: Hutchinson. 



CHAPTER 



26 



Polymorphism 



If natural selection constantly causes the elimination of 
the less fit, in time a population might be expected to consist 
solely of the best adapted type. In reality, such a situation seldom 
if ever exists, for despite the constant pressure of natural selec- 
tion, wild populations continue to have considerable genetic vari- 
ability, a fact already discussed in an earlier chapter. Now we 
must consider in more detail how this variability is maintained. 

A population is said to be polymorphic when two or 
more distinct types of individuals coexist in the same breeding 
population. Ford has limited this definition further by saying that 
the forms must exist in such proportions that the rarest is not 
being retained in the population merely by recurrent mutation. 
However, this added restriction is not particularly useful, for it 
presupposes a knowledge of the mutation rates in natural popula- 
tions that is rarely available, and it cannot easily be applied except 
by inference. Polymorphism is used with respect to what we have 
earlier called discontinuous traits rather than for continuous varia- 
tion. These traits may be morphological, in which case they are 
generally controlled by two or more alleles of a gene of major 
effect, and therefore present no difficulty in classification. They 
may also be chromosomal; the various inversion types in Dro- 
sophila pseudoobscura mentioned earlier represent a case of chro- 
mosomal polymorphism. Furthermore, human populations are not 
only polymorphic for many morphological traits, but they are also 
polymorphic for the blood groups. Thus, whether polymorphism 
is open to study depends to some extent on whether suitable 

249 



250 • THE MECHANISM OF EVOLUTION 

methods for its detection have been devised. Chromosomal and blood group dif- 
ferences are clearly discontinuous, but they became subjects of research only after 
cytological and serological techniques for their detection had been developed. 

The definition is intended to exclude such differences as are observed 
between geographical races. The differences between members of the same species 
that belong to different breeding populations living in separate areas are said to 
be polytypic. Different races of birds may overwinter in the same region and thus 
coexist for a time, but this situation cannot be termed polymorphism, for the 
races are still members of separate breeding populations. More will be said later 
about the origin of polytypic differences in races, but at this point we shall con- 
centrate on polymorphism. 

In the chapter on selection we have already seen that an equilibrium 
may be established between mutation pressure and selection pressure. Thus the 
polymorphism observed in a population may be due simply to the balance be- 
tween the forces of mutation and selection. Furthermore, the Hardy- Weinberg 
equilibrium is established when the various genotypes all have the same selective 
value or are adaptively neutral. Proof of adaptive neutrality is virtually impos- 
sible since a demonstration that no selective advantage exists under one set of 
genetic and environmental conditions is no proof that it might not exist under 
somewhat different circumstances. The possible variations in conditions being 
almost limitless, pursuit of adaptive neutrality is like chasing a will o' the wisp. 
Nevertheless, it remains a possibility not to be ignored, especially since the genes 
may be neutral except under quite specific conditions. However, many cases of 
polymorphism are adaptive and clearly involve more than these relatively simple 
types of equilibria. For this reason polymorphism has assumed a significant place 
in evolutionary studies. 

Transient Polymorphism 

Two additional types of polymorphism have been identified, transient 
and balanced. Transient polymorphism exists during the period when a new or 
previously rare mutant becomes advantageous and spreads through the popula- 
tion. During its spread, an obvious but transient polymorphism will exist. It is 
transient because the new. form will eventually (except for mutation) replace the 
old. Balanced polymorphism exists when selection .actively maintains more than 
one type in a population, A variety of types of balanced polymorphism has 
been discovered. Because of their very nature, balanced polymorphisms will be 
more common than examples of transient polymorphism. 

The most carefully studied case of transient polymorphism is the phe- 
nomenon known as industrial melanism, which has been observed in at least 
70 species of moths in England and on the continent of Europe. Although other- 
wise they may be quite different, all of these moths normally rest in exposed 



POLYMORPHISM • 251 



places, depending for protection on their cryptic coloration, a mottled pattern 
that blends in with a background of bark or lichen. The industrial revolution of 
the past century and a half has had a profound effect on the countryside in 
industrial regions. The smoke and soot from thousands of chimneys have coated 
trees and shrubs for miles around. As a consequence the background on which 
the moths now must rest in industrial areas is much darker than it was over a 
century ago. A remarkable change in these species has led to the replacement of 
the typical mottled forms by much darker melanic forms in the industrial areas. 
In some species (for example, the peppered moth, Bis ton betularia) the fre- 




Fig. 26-1. Left: dark and light forms of the peppered moth (Bis ton betularia) 

on the trunk of an oak at the industrial city of Birmingham, England. Right: 

dark and light forms of the peppered moth on the lichen-coated trunk of an oak 

in an unpolluted region. (Courtesy of Kettlewell.) 

quency of the melanic types has reached over 95 percent in many populations. 
Kettlewell has shown that in industrial regions the melanic type is much less 
likely to be taken by birds than the typical mottled moths, but that in unpolluted 
country the melanic form is quite conspicuous and is subject to heavier predation 
by birds than are moths with the typical pattern (see Fig. 26-1). There are also 
indications that the melanic moths may differ in viability or behavior from the 
typical form. 

In virtually all of the species the transition has been due to the increase 
in frequency of dominant mutant genes for melanism even though recessive 
mutants and systems of multiple factors are also known to cause increased 
melanin production in at least some of these species. Since the various kinds of 



252 • THE MECHANISM OF EVOLUTION 

black moths were all rather rare prior to the industrial revolution, it is quite 
clear that natural selection has operated specifically to bring about this pheno- 
typic transition through the dominant mutants rather than through some other 
genetic mechanism. Although other reasons for the utilization of dominants have 
been suggested, the most obvious was given many years ago by Haldane, who 
showed that in a randomly mating population a rare dominant will increase in 
frequency when favored by selection much more rapidly than will a rare recessive 
or a rare polygenic system. The reasons for this fact are quite simple. All of the 
dominant mutants are exposed to selection and hence when selection pressure 
shifts to favor the dominants, half of their progeny will carry and express the 
dominant in the next generation and will again be favored by natural selection. 
Rare recessive individuals, though also favored by selection because of their 
phenotype, will seldom leave progeny like themselves since most of their matings 
will be with wild-type individuals, and the favored recessive mutant will be 
submerged in the heterozygous condition in the population until by chance in 
future generations two recessives again combine in a single individual. Selection 
will ordinarily work even less effectively to increase the frequency of rare favor- 
able polygenic systems, since they are constantly being broken up by genetic re- 
combination. Thus, it is not at all surprising that even though various genetic 
mechanisms causing melanism must have been available in these species, the one 
almost invariably selected was the dominant mutant. 

Although industrial melanism in moths is probably the most closely 
studied case of adaptive polymorphism involving dominant mutant genes, many 
other examples of polymorphism involving dominants to the wild type can be 
cited. Melanism in the hamster (Cricetus crzcetus), color patterns in the grouse 
locust (Apotettix eurycephalus), in the platyfish (Platypoecilus maculatus), in 
ladybird beetles (Coccznellzdae), and in frogs {burnsi and kandiyohi mutants in 
Rana pipiens) are all controlled by dominant genes and have relatively high 
frequencies in natural populations. In domesticated plants such as barley, oats, 
wheat, flax, cotton, cabbage, and tomatoes many cases of disease resistance con- 
trolled by simple dominant mutations can be cited. Furthermore, resistance to 
subtertian malaria in man has been shown to be increased in individuals hetero- 
zygous for the sickle cell gene. All of these examples — and more could be cited 
— suggest that dominant mutations may play a significant role not only in poly- 
morphism but in evolution as well. 

The Origin of Dominance 

Thus far, we have taken dominance and recessiveness more or less for 
granted although we have discussed the fact that dominance is not exclusively a 
property of a particular gene, but may be modified by the rest of the genotype 
and by both the internal and external environment in which the gene functions. 



POLYMORPHISM • 253 

At this point it seems advisable to raise the question of the origin of dominance. 
Several hypotheses have been advanced, and it seems likely that no one theory is 
correct and the others wrong, but rather that each contains some elements of 
truth. 

Bateson and Punnett were the first to suggest a theory of dominance 
when they proposed that the recessive condition was due to the absence of the 
dominant. This simple presence-absence concept became untenable after the dis- 
covery of dominant effects due to deficiencies, of reverse mutations from reces- 
sive to dominant, and of multiple alleles. 

Fisher pointed out that the great majority of mutants that occur are 
deleterious and are recessive to the "normal" or "wild-type" alleles found in 
natural populations, and he thus framed the question in terms of the origin of 
dominance of wild-type genes. He further noted that mutations are recurrent and 
frequent enough so that a given mutant will be regularly reintroduced into a 
population even though it is deleterious. He assumed that the very first time a 
particular mutation occurs, the heterozygote will be phenotypically intermediate 
between the two homozygotes. Dominance will then arise as the result of the 
selection of modifying factors at other loci that push the expression of the inter- 
mediate heterozygote toward that of the homozygous wild type. 

Several difficulties in this theory should be pointed out. The assumption 
of an initially intermediate heterozygote is in a sense gratuitous, for it is actually 
part of what must be proven. Furthermore, the theory offers no adequate expla- 
nation for the appearance of the occasional recurrent deleterious mutant that is 
dominant to the wild type. Wright has also estimated that heterozygotes will be 
so infrequent and the selective advantage so slight that the selection pressures 
will be too small to be a controlling factor in the fixation of modifiers. In addi- 
tion, the modifiers will have other primary effects of their own, and their ulti- 
mate frequency will depend more on the action of selection with respect to these 
primary effects than it will on their effects on the dominance of some other gene. 

As an alternative to Fisher's theory of modifiers Wright suggested a 
physiological theory of dominance. He noted that the normal or wild-type genes 
are functional, but deleterious mutants represent a partial or complete inactiva- 
tion of the gene. Dominance then results because the wild-type allele, which is 
active, will be expressed in the presence of the deleterious mutant, which is not. 
The genes are presumed to control the formation of enzymes, which catalyze 
chemical reactions in living things. The rate of these enzymatic reactions depends 
on both the concentration of the enzyme and that of the substrate. If a single 
normal gene in a heterozygote produces enough enzyme for a reaction to proceed 
at the maximum rate possible, the heterozygote will resemble the homozygote, 
and dominance will be complete. If, on the other hand, it does not produce 
enough enzyme, dominance will be incomplete, but the greater the activity of the 
gene, the more the heterozygote will resemble the homozygote. 



254 • THE MECHANISM OF EVOLUTION 

Haldane proposed that dominance resulted from the selection of the 
more efficient wild-type alleles from among a group of different wild-type alleles 
or isoalleles. Since individuals heterozygous for the more active allele would be 
more like the normal homozygote, they would have a selective advantage in 
heterozygotes, and the more active allele would be favored by selection over the 
less active type. Thus he argued that selection would favor the allele that had a 
safety factor of at least two in enzyme production so that a single gene could 
perform the task ordinarily done by two. This theory, like Wright's, is essentially 
a physiological theory of dominance. 

A final theory, developed by Plunkett and Muller, again involves the 
selection of modifiers. Unlike Fisher's idea, however, selection is directed, not 
primarily at the infrequent heterozygotes, but at the wild-type homozygotes. 
Those modifying factors are selected that tend to stabilize the wild-type pheno- 
type under all sorts of environmental and genetic stresses. Under this hypothesis, 
modifiers are selected not just for their ability to suppress the harmful effects of 
an occasional deleterious mutant, but rather to build up a safety factor for the 
wild type. 

From the wealth of theories it is clear that the question of the origin of 
dominance has not yet been finally resolved. Experimental evidence can be cited 
in support of both the physiological and modifier theories. There is no question, 
for example, that dominance can be shifted by the selection of suitable modifiers. 
Nevertheless, it is also true that different wild-type alleles may show different 
degrees of dominance in heterozygotes. The theories are not mutually exclusive, 
for it is quite conceivable that mutants may occur that are favorable and domi- 
nant from the outset and are immediately favored by selection. However, if such 
mutants are not available, selection may be forced to work with the genetic mate- 
rials at hand to increase the dominance of existing mutants through modifiers at 
other loci. 

Balanced Polymorphism 

Balanced polymorphism may arise in a number of different ways. If the 
rarer form were always at a selective advantage, adaptive values would change as 
frequencies changed. A rare form favored by selection would lose this selective 
advantage as it became more common, until at high frequencies it would be at a 
disadvantage. In this way selection would tend to damp any oscillations in gene 
frequency before they led to the extinction of one allele, and a balanced situation 
would be maintained. Such a situation might arise as a result of the feeding 
habits of predators that tend to take the common forms of their polymorphic 
prey but overlook the rare ones. 

In the twin-spot ladybird beetle (Adalia bipunctatOi) changing selection 
pressures of a somewhat different kind are responsible for still another type of 



POLYMORPHISM • 255 

equilibrium. The red phase increases in relative frequency during the winter, but 
the black phase increases during the summer. As a result of the seasonal shifts in 
adaptive value, neither type is eliminated. Similar seasonal shifts in the frequency 
of inversion types in Drosophila pseudoobscura indicate that balanced poly- 
morphism is a device by which this species, too, adapts to seasonal changes. 
Seasonal polymorphism is more apt to be observed in species with a short genera- 
tion length. 

A rather unusual type of polymorphism is exemplified by the T locus in 
mice. A number of distinct alleles have been found in different wild populations 
that in the homozygous condition cause sterility or even lethality but have no 
visible effect on the phenotype of heterozygotes. Mendelian segregation in 
heterozygous females is normal, so that eggs bearing mutant and normal genes 
are produced in equal numbers. However, in heterozygous males, segregation is 
highly abnormal, for up to 95 percent of the sperm cells carry the deleterious 
mutant. Under these circumstances, the increase in frequency of the mutant that 
would otherwise occur is checked or held in balance by the lethal or sterile effects 
of the gene. Comparable examples have been described in Drosophila under the 
term "meiotic drive." Many questions remain to be answered about what appear 
to be most peculiar and anomalous situations. 

Any system whereby mating between individuals of unlike genotype is 
encouraged or enforced leads to the establishment of a stable polymorphism. In- 
compatibility systems in plants are a case in point. Some species such as red 
clover (Trifolium pratense) have a series of multiple self -sterility alleles, S lf S 2 , 
5*3, S±, etc. Pollen that carries any particular allele will fail to fertilize the ovules 
of any plant carrying the same allele. Thus S t pollen will successfully fertilize 
ovules in S 2 S 3 , S 2 S 4 , and S 3 S 4 plants but not in S^z,- S1S3, or S t S 4 plants. Self- 
fertilization is therefore impossible, and furthermore no homozygotes can be 
formed. 

The Pin-Thrum situation in the primrose {Primula vulgaris) is com- 
parable but differs in some respects. Pin flowers have a long style with the stigma 
at the mouth of the corolla tube of the flowers and the anthers half-way down 
the tube. In Thrum flowers the positions of anthers and stigma are reversed as 
compared to Pin. This difference ordinarily behaves as if controlled by a single 
locus, with Pin being the homozygous recessive (pp) and Thrum the hetero- 
zygote (Pp). The pollen tube formed by Pin pollen grows only very slowly on 
Pin, but Thrum pollen on a Thrum stigma forms no pollen tube at all. Since 
Thrum is a heterozygote, its pollen is of two types. Therefore, the pollen be- 
havior must be determined, not by the genotype of the pollen itself as with the 
self-sterility alleles, but by the genotype of the Thrum parent, for p pollen from 
a Pin plant will grow down the style of a Thrum (Pp) plant, but genetically 
similar p pollen from a Thrum plant will not. 

In animals, nonrandom mating has occasionally been reported in which 



256 • THE MECHANISM OF EVOLUTION 

unlike individuals are more apt to mate than individuals of like genotype. If 
negative assortative mating of this kind actually does occur, it too would result in 
balanced polymorphism, for individuals of the rarer type would have a greater 
likelihood of obtaining mates. This case actually represents still another way in 
which selection intensity would be related to gene frequency. 

Although this category is seldom included in discussions of polymorph- 
ism, it is worth pointing out that any species with separate sexes is polymorphic 
in every sense of the word. In most cases this polymorphism is chromosomal as 
well as phenotypic, and cross fertilization is mandatory. In addition to the 
primary differences between the sexes, there are many secondary sexual charac- 
ters. The adaptive value of these traits in many cases seems quite apparent, but 
much remains to be learned about these adaptive values, their mode of origin by 
selection, and the genetic mechanisms controlling them. 



Heterosis and Polymorphism 

The final mechanism of balanced polymorphism to be discussed is the 
situation in which the heterozygote is more fit than either homozygote. In other 
words, heterosis may also serve as a means of maintaining balanced polymorph- 
ism. The most extreme case of this sort is a balanced lethal system. If linkage is 
close or crossing over is in some way suppressed, only Ab/aB progeny will re- 
sult from Ab/aB heterozygous parents, for the Ab/Ab and aB/aB homozygotes 
will die owing to the homozygous recessive lethals (bb or aa). Individuals of 
the Ab/aB type will breed true in spite of being heterozygous. 

Overdominance will also lead to a balanced heterozygous system. In 
this case only a single locus need be involved, and the homozygotes may be only 
slightly inferior to the heterozygote. When the heterozygote (Aa) is superior, 
selection, rather than tending toward homozygosity for a favored allele, will 
favor the heterozygotes, and hence will produce a stable equilibrium at the gene 
frequencies that confer optimum fitness on the entire population. These fre- 
quencies are determined by the relative fitness of the two homozygotes. If the 
fitness of Aa is set equal to 1, of A A equal to (1 — s x ), and of aa equal to 
(1 — j- 2 ), then 



* V 1 - s x f - stf 
and at equilibrium Ap = and s\p = s 2 q 

Solving this equation, 

A J"2 



J-l + J-2 









POLYMORPHISM 


For example, 


if Aa = 1 








AA = 1 


- ji = .8 


C'l = -2) 




aa — 1 


- j-2 = .4 


(i-2 = -6) 


Then 


A 


6 , = -75 





257 



The best example of single gene heterosis responsible for balanced 
polymorphism comes from man. The sickle cell gene (Hb s ) produces an ab- 
normal hemoglobin and in homozygous condition causes sickle cell anemia, a 
debilitating disease that is usually fatal. This gene has a surprisingly high fre- 
quency in some parts of the world. In these areas malaria is endemic, and it has 
been found that the heterozygotes (Jib s /Hb a ) for the sickle cell gene are signifi- 
cantly more resistant to subtertian malaria than are the homozygotes (Hb a /Hb a ) 
for normal adult hemoglobin. Thus where malaria is prevalent, the heterozygotes 
are better adapted than the homozygotes, which are apt to die either from anemia 
on the one hand (Hb s /Hb s ) or malaria on the other (Hb a /Hb a ). 

Probably the most thoroughly studied case of heterozygote superiority 
is that of inversion heterozygotes in Drosophila. In some species of Drosophila 
(for example, D. pseudoobscura, D. persimilis, D. miranda, D. robusta, and D. 
willistorii) two or more inversions may occur with high frequency within a 
single breeding population. The seasonal shifts in frequency of inversion types 
have already been mentioned, but even more significant is the fact that the inver- 
sion heterozygotes show hybrid vigor or superior fitness as compared to the in- 
version homozygotes even though their external appearances are similar. The 
implication is clear that the different inversion types must differ to some extent 
in their gene contents. Since crossing over is restricted in inversion heterozygotes, 
the development of these differences is not surprising. This is not to suggest, 
however, that all chromosomes of, say, the Standard type in a breeding popula- 
tion of D. pseudoobscura have the same gene contents, but merely that two 
Standard chromosomes from the same population will generally be more alike 
than will a Standard- and an Arrowhead-type chromosome drawn from the same 
population. Since the block of chromatin within an inversion will be isolated 
from recombination with other inversions, the gene complex within an inversion 
will be subject to selection as a unit. These gene complexes can thus be expected 
to differ from each other in both gene contents and adaptive value. Furthermore, 
it has been postulated that selection will also operate to favor those combinations 
of genes in each inversion type that confer maximum heterosis or fitness when in 
heterozygous combination with another inversion, since inversion heterozygotes 
are ordinarily more common than inversion homozygotes. Thus, in addition to 
its adaptive value as a homozygote each inversion type may have an adaptive 



258 • THE MECHANISM OF EVOLUTION 

value as a heterozygote, or will be "coadapted" to the other gene complexes in 
the population. 

One additional observation about these inversion heterozygotes should 
be noted. In general, the heterozygotes are phenotypically more stable or show 
less variation under environmental stress than do the corresponding homozygotes. 
Furthermore, a heterozygous population is better able to adapt to changing en- 
vironmental conditions without major disruptions than is a relatively homozygous 
population. These two concepts, in some ways related, have been widely dis- 
cussed under the terms "developmental homeostasis" and "genetic homeostasis" 
respectively. 

That the different inversions do differ in adaptive value is indicated by 
their seasonal and altitudinal shifts in frequency. In population studies in Cali- 
fornia, for example, the Standard type in D. pseudoobscura increased in fre- 
quency as the weather became warmer, reaching a maximum during the hot 
summer months. Populations sampled at different altitudes formed a cline with 
Standard having a low frequency at high altitudes and increasing in frequency 
with lower elevation. Since altitude also provides a temperature gradient, the 
Standard gene complex in this region appears to be better adapted to warmer 
temperatures than the other inversions in these populations. Here, as demon- 
strated previously, the relative frequencies in this balanced polymorphic system 
will be determined by the relationship between the selection coefficients of the 
homozygous types. 

Samples taken over the wide geographical range of a species may also 
show shifts in the frequency and kinds of the different third-chromosome inver- 
sions. These differences undoubtedly reflect changing adaptive requirements 
under different ecological conditions, but they may also reflect historical events, 
in the sense that different chromosomal mutations may have occurred in different 
parts of the range. Since selection must operate within the framework of the 
available variability, some of the geographic variation in inversion types may 
have arisen in this way. 

The amount of inversion heterozygosity has been found to vary greatly, 
usually being maximal toward the center of the range of a species and decreasing 
toward the periphery. One theory proposes that chromosomal polymorphism per- 
mits the species to exploit a greater variety of ecological niches than would other- 
wise be open to it. Thus, at the center of the range the species is presumed to 
be highly successful, exploiting a number of different niches, but at the limits of 
the range the environment is marginal for the species and a minimal number of 
niches are habitable. 

Another hypothesis is that the primary function of inversion hetero- 
zygosity in natural populations is related to its effects on recombination. In the 
central populations, with a high frequency of inversion heterozygosity, the 
amount of possible genetic recombination will be considerably restricted. Selec- 



POLYMORPHISM • 259 

tion will tend to favor heterozygotes with superior general vigor, and adaptation 
will be achieved through heterosis. This type of adjustment is only feasible in 
large populations, for it is made at the expense of the production of homozygotes 
of low fitness. Any device, such as an inversion, that would tend to reduce the 
frequency with which relatively unfit homozygotes are formed will have an im- 
mediate selective value because it will minimize the cost of maintaining heterosis 
in the population. When adaptation via heterosis occurs, the population can meet 
rather drastic environmental changes with relatively minor adjustments in its 
heterotic genetic system; it is said to be "heterotically buffered." However, such 
a system imposes a considerable limitation on the possibilities for future evolu- 
tionary change. 

On the other hand, in marginal populations, small in numbers and rela- 
tively isolated, inversion heterozygosity is low and genetic recombination 
relatively unrestricted. Under these circumstances selection will tend toward the 
ultimate fixation of those genes conferring superior fitness. It is in these popula- 
tions, it is argued, that the evolutionary changes occur that lead to genetic diver- 
gence and ultimately to the formation of new subspecies and species. 

Although a great deal of very fascinating work has been done on 
chromosomal polymorphism in Drosophila, it seems likely that there is still much 
to be learned. For example, why should inversion heterozygosity be so common 
in some species of the genus Drosophila but rare or absent in other species such 
as D. melanogaster and D. virilis, which are widely distributed and highly suc- 
cessful in exploiting a variety of ecological niches? A most interesting observa- 
tion made some years ago by Dubinin in Russia showed that the frequency of 
inversion heterozygosity in D. funebris was related to the degree of industrializa- 
tion of the area in which the population lived. Thus, populations in large urban 
areas showed a high degree of inversion heterozygosity, but the frequency de- 
clined in suburban and small-town populations until it was virtually zero in rural 
districts. This difference may well be related to the number of adaptive niches 
available in urban as compared to rural areas, but it may also reflect the effect of 
differences in population size of the flies or of passive transport of flies into the 
cities. Only further study can resolve these questions. 

The material already presented should suffice to illustrate some of the 
complexities related to polymorphism, but still other aspects of this subject 
may be mentioned. Many instances of mimicry, for example, also involve poly- 
morphism, sometimes affecting just one sex and not the other. Environmental 
factors may also induce polymorphic differences; pupa case color in certain 
species of butterflies is related to the type of background on which chrysalis 
formation occurs. Green pupae are more common on the green leaves of plants 
whereas brown pupae are more frequent if the pupae are formed on the brown 
stems. These differences reflect a delicate adjustment between the genotype and 
the environment. Still other polymorphisms observed in the field may be due to 



260 • THE MECHANISM OF EVOLUTION 

the ability of the individual organism to change its color to match its back- 
ground, an ability fairly common in the animal kingdom. Tree frogs among the 
amphibians, the chameleon among the reptiles, and the cuttlefish, a molluscan 
invertebrate, are familiar examples of species with great capacity in this respect. 
In man, polymorphisms of many kinds may be observed, but their sig- 
nificance is usually unknown. In the past, the blood groups were frequently re- 
ferred to as adaptively neutral traits, but the discovery of the relation between 
the sickle cell gene and malarial resistance, and between other blood group genes 
in the ABO system and the incidence of stomach cancer and duodenal ulcer indi- 
cates that this is a hazardous assumption. Other cases present problems of 
particular interest and importance. Both schizophrenia and diabetes have an inci- 
dence in human populations of about 1 percent despite the fact that the repro- 
ductive rate of affected persons in the past must have been significantly lower 
than that of unaffected individuals. Since an underlying genetic basis has been 
demonstrated for both illnesses, the high frequency of diabetes and schizophrenia 
suggests the existence of balanced polymorphism, but the possible mechanism 
remains unknown. The study of polymorphism has been an exceptionally fruitful 
area of research for students of variation and evolution, and these and many 
other problems suggest that it will continue to be so for some time to come. 



SUMMARY <- 



A polymorphic population contains two or more distinct 
types of individuals. Not only genie but chromosomal polymorph- 
isms have been discovered. Polymorphism may result from the 
Hardy-Weinberg equilibrium or from the balance between the 
opposing forces of mutation and selection. Of even greater inter- 
est are transient and balanced polymorphism. The most thoroughly 
studied case of transient polymorphism, industrial melanism, has 
shown that in industrial regions in Europe, the light, mottled 
pattern of many moths has been almost completely replaced in a 
matter of decades by a darker, melanic form, better adapted to 
the new background. Numerous examples of polymorphism in- 
volving dominant mutants are known, and there are various 
theories of the origin of dominance. Balanced polymorphism may 
be due to a number of conditions, among them shifting selection 
pressures and selection favoring the heterozygotes over both 
homozygotes. The study of balanced polymorphism has loomed 
large in recent work on the nature and origin of species, and it 
remains a fertile field for research. 



POLYMORPHISM • 26l 



SUGGESTED READING 

Cold Spring Harbor Symp. Quant. Biol., Vol. 20, 1955. "Population genetics." Long 

Island Biological Assoc, New York. 
Dobzhansky, Th., 1951. Genetics and the origin of species, 3d ed. New York: 

Columbia University Press. 
Sheppard, P. M., 1958. Natural selection and heredity. London: Hutchinson. 



CHAPTER 



27 

Genetic Drift 



Thus far in our discussions of the genetics of populations 
we have been making the implicit assumption that the populations 
were infinitely large. In actuality natural populations are, of 
course, finite in size and may be quite small. JEven when the total 
population is very large, if it is divided into numerous small, iso- 
lated, breeding populations, the dynamics of the changes in gene 
frequency will be determined by the forces operating in each 
small population independent of the rest. If there is some migra- 
tion between the different breeding populations, the evolutionary 
course of the entire species will be tied together in a very complex 
manner that depends not only on mutation pressure and the selec- 
tion pressures within and between populations, but also on the 
size of the various breeding populations and on the amount of 
migration between them. We have already considered the effects 
of mutation and selection. Now we must discuss the effect of 
population size on an isolated population, before going on in a 
later chapter to treat migration or gene flow. 

The total number of individuals in a species, without 
reference to the way in which the species may be subdivided into 
breeding populations, gives little indication of the possible effects 
of population size on gene frequency changes. Similarly, a simple 
census of the number of individuals in a single population may 
not be a true index of the effective breeding size of the popula- 
tion. Some species, for example, undergo drastic periodic seasonal 
fluctuations in numbers. A census taken in the fall may indicate a 
size in the hundreds of thousands or even millions for an insect 



262 



GENETIC DRIFT • 263 

population in the temperate zone. However, if only a fraction of 1 percent of 
these insects survive the winter, the characteristics of this population will largely 
be determined by this handful of survivors rather than by the much larger num- 
ber at the population peak. 

We have already seen in the discussion of the Hardy-Weinberg equilib- 
rium that in a large, randomly mating population, in which there is no mutation 
or selection, gene frequencies will remain constant. However, if the population 
is small, gene frequencies will tend to fluctuate purely by chance, and the smaller 
the population, the greater the fluctuations are apt to be. These random changes 
in gene frequency are said to be due to genetic drift. The gene frequencies in a 
small population will continue to fluctuate until one allele is lost and the other 
fixed. Subsequently, the population will remain homozygous unless a new muta- 
tion appears. 

— ^'"The basis for genetic drift is to be found in the process of sampling. 
In order to understand the relation between population size and drift, we must 
understand certain elementary principles of sampling. If the gene A is repre- 
sented by a black marble and its allele a by a white one, then all of the gametes 
produced by a population can be represented by a large bowl full of marbles, 
with the black marbles representing the proportion of A genes in the gametes. 
Obviously not all of the gametes produced will go to form the next generation, 
for many, especially the sperm, will not take part in fertilization, and many of 
the fertilized eggs will not survive to maturity. Thus, the gametes that actually 
give rise to the next generation can be represented by a handful of marbles taken 
from the bowl. If there are equal numbers of A and a genes in the gametes, the 
gene frequency of A is 50 percent. However, in a handful of marbles taken at 
random, it is unlikely that the numbers will be exactly equal. Similarly, because 
of the random nature of meiosis and fertilization, the numbers of dominant and 
recessive genes may not be equal. The principles involved in estimating how 
large the deviations from equality may be are much the same as those used in 
estimating the expected numbers of heads and tails with a tossed coin. If you 
tossed a penny four times, you would probably not be surprised if you got three 
tails and one head. In fact, it can be estimated that such a result would be ex- 
pected 25 percent of the time when four tosses are made. The probabilities for 
various combinations of heads and tails on four tosses are calculable from ex- 
pansion of the binomial (a + &) 4 , where a = y 2 = the probability of heads, 
and b = ]/ 2 = the probability of tails. The complete expansion is : 







3 heads 


2 heads 


1 head 




composition of sample 


4 heads 


1 tail 


2 tails 


3 tails 


4 tails 


proportion of heads 


1 


.75 


.50 


.25 





frequency 


a" 


4a z b 


6a 2 b 2 


4ab* 


b± 


probability of sample 












of above type 


% 


He 


% 


Vm 


He 



*- 



264 • THE MECHANISM OF EVOLUTION 

Thus, less than half the time (% 6 ) would you expect to get equal numbers of 
heads and tails, or of black and white marbles, or of dominant and recessive 
genes in samples of four drawn from a source of supply in which each type has 
an equal frequency. In terms of gene frequencies, it is clear that there is a sizable 
chance that the frequency of A will shift either to .75 or .25 or that A may 
become either fixed or lost from the population. 

However, if you tossed a penny 10,000 times, you would be very sur- 
prised if you got 7500 tails and only 2500 heads, and rightly so, even though 
the ratio of heads and tails is the same as for 3 tails and 1 head. Your more or 
less instinctive reaction can be borne out statistically, for the standard error of a 

ratio for large samples equals . / P X 1 or in this case A / (°- 30 ) (0.50) 

\ n \ 10,000 

= 0.005. Thus with 10,000 tosses, expectations are for 5000 heads, with a 
standard error of 50. Since the chances are less than 1 in 100,000 that a sample 
will diverge from its source by as much as four times its standard error, even a 
ratio of 5200 tails to 4800 heads would be extremely improbable. From this line 
of reasoning, it should be clear why random fluctuations in gene frequency tend 
to be larger, the smaller the sample of genes that gives rise to the next generation. 
One further point to note is that the sample of genes that goes to form 
the first generation will then in its turn generate the new supply of gametes from 
which the genes of the second generation will be drawn. Therefore, if sampling 
fluctuations have resulted in frequencies of A and a other than 0.5, the sampling 
situation is likely to be somewhat different in the next generation than it was in 
the preceding one. If, for example, 1 white and 3 black marbles were drawn at 
random from a bowl containing equal numbers of black and white, the new bowl 
of marbles from which the next sample must be drawn would contain, not equal 
numbers of black and white, but % black and only y^ white. Over a number of 
generations, sampling fluctuations may have a cumulative effect and gene fre- 
quencies may diverge considerably from their initial frequencies, hence the name 
"genetic drift." As a result of random genetic drift a new mutant may occa- 
sionally spread through a small population until it becomes homozygous or fixed 
in the population, but more often random drift will lead to the loss of the new 
allele before it has even had a chance to spread. 



Effective Size of Populations 

The effects of genetic drift have been estimated under various condi- 
tions, but a special case of rather general interest will suffice to give some indica- 
tion of the relation between population size and genetic drift. In a population of 
moderate size with equal numbers of males and females mating at random, the 
rate of decay of the variability or the rate of decrease in heterozygosis is approxi- 
mately equal to 1/2N. Here, N is the effective size of the breeding population 



GENETIC DRIFT • 265 

rather than the total number of individuals in the population, for many will not 
survive to maturity and among those that do, not all will leave offspring. Thus, 
the actual progenitors contributing genetically to the next generation may be con- 
siderably fewer in number than the total number of individuals living in the 
population at any one time. Furthermore, the breeding population may be larger 
than the so-called effective size of the population. The breeding population will 
equal the effective population when equal numbers of males and females are 
mating at random and contributing equally to the next generation. However, if 
the numbers of males and females are unequal, the effective size will depend to 
a large extent on the sex which is fewer in number. Thus, for example, in a 
flock of chickens with a few roosters serving a large number of hens, the effec- 
tive size of this population will approximate four times the number of roosters 
rather than the total number of breeding individuals. Similarly, in a population 
undergoing periodic expansion and contraction in numbers, the effective N will 
be much closer to the minimum number than to the maximum. As a simple ex- 
ample of the effect of drift, if N were 20, 1/2N or 1 out of 40 heterozygous loci 
on the average would be expected to become homozygous in the next generation. 
It can be seen that, continued over a number of generations, genetic drift would 
not only cause fluctuations in gene frequency but also would increase the amount 
of homozygosity in the population. 

Cases to illustrate the effects of genetic drift can be drawn from man. 
American Indian tribes are known to have formed rather small, isolated, mating 
populations in recent times and are thought to have formed such units ever since 
they first migrated to America. Human populations in other parts of the world 
do not ordinarily consist of such small mating isolates. It is significant therefore 
that whereas the_.iiequency of the gene producing the A substance of the ABO 
blood group system ranges in the rest of the world from about 15 percent to 
45 percent, in_ American Indian tribes it ranges from as low as 1 or 2 percent in 
some tribes to as high as 80 percent in the Bloods and the Blackfeet. A study of 
a genetic isolate based on religion has also produced some interesting data. The 
Old German Baptist Brethren, or Dunkers, form a community of about 300 per- 
sons in Franklin County, Pennsylvania, but the effective size of this population 
has been estimated to be only about 90. This group was compared for a number 
of traits both with the population of the German Rhineland, their place of ori- 
gin, and also with the population in the United States among whom they live 
and from whom they have drawn a small fraction of their genes by intermarriage. 
The analysis showed quite clearly that this community had 'come to differ signifi- 
cantly from the populations both in Germany and the United States in several 
but not all of the traits studied — exactly the result that might be expected with 
genetic drift. The evidence, therefore, is highly suggestive that genetic drift does 
play a considerable role in determining gene frequencies in small isolated human 
populations. 



266 • THE MECHANISM OF EVOLUTION 

Genetic Drift and Evolution 

Considerable discussion has arisen over the evolutionary significance of 
genetic drift. The debate has hinged, not so much on whether genetic drift can 
occur, but rather on whether, even if it does occur, it has any long-range impor- 
tance in evolution. Given the facts of Mendelian inheritance, there seems little 
reason to doubt that random genetic drift can take place, and if this is so, it then 
seems highly probable that in particular instances or under certain circumstances 
it has played a role in evolution. The fate of most small breeding populations is 
undoubtedly extinction, due either to the vicissitudes that affect any natural 
population, or to the populations' inability to adapt to changing conditions be- 
cause of their low variability, or simply to loss of identity by interbreeding with 
members of other, larger populations. The question still remains as to the evolu- 
tionary role of the occasional small, divergent population that survives. The 
available data, at best not too abundant, have frequently been analyzed from 
only one point of view. For example, the "drifters" have sometimes assumed that 
apparently random gene frequency differences between different breeding popu- 
lations of the same species are de facto evidence for genetic drift, and have made 
no attempt to determine whether these differences are in any way adaptive. On 
the other hand, the "selectionists" may consider that by proving that selection is 
operating in a population they have thereby excluded the possibility of genetic 
drift, or they may fail to make the essential distinction between effective size and 
population number. Furthermore, drift seems likely to be of greater significance 
in some kinds of species than in others. Top carnivores, for instance, which are 
relatively very few in number and apt to be widely scattered, might well be more 
likely subjects to investigate for the effects of drift than some of the species 
studied thus far. 

In actual populations, natural selection undoubtedly functions at all 
population sizes, small as well as large. Therefore, it may be expected that 
genetic drift in the absence of selection will rarely be found. When selection as 
well as genetic drift is operative, both will tend to cooperate, and the deleterious 
genes in small populations will be eliminated more rapidly than in large popula- 
tions in which selection alone is effective. The reason is that the less frequent 
allele in a population has a somewhat greater probability of decreasing than of 
increasing in frequency under genetic drift. Since the constant pressure of selec- 
tion will keep the deleterious gene at a low frequency, the net effect of selection 
plus drift is to increase the rate of elimination of deleterious genes. Natural 
selection is the controlling factor in the evolution of large populations r .which 
usually remain quite heterozygous and hence retain considerable variability, 
either actual or potential. In small populations, the combined effect of natural 
selection, genetic drift, and the greater likelihood of inbreeding is to raise the 
level of homozygosity and thus lower the amount of variability in the population. 
For this reason, small populations may lose their ability to adapt to changing 



GENETIC DRIFT • 267 

conditions and become extinct. However, numerous small populations may also 
come to diverge from each other both as a result of different selection pressures 
and the chance events stemming from mutation, genetic drift, and inbreeding. 
Hence each population may be regarded as a separate evolutionary experiment, 
and even though the fate of most of them is extinction, the possibility for rather 
rapid evolution in novel directions under these circumstances cannot be ignored. 



^SUMMARY 



Changes in gene frequencies may occur in small popula- 
tions as the result of random genetic drift. In essence, genetic 
drift is a consequence of drawing a small random sample of 
gametes to form the next generation. This sample, which by 
chance may differ in gene frequency from the gene frequencies 
in the parents, then becomes the new gene pool from which the 
gametes for the next generation are drawn. In this way, numer- 
ous unpredictable changes in gene frequency within a population 
may take place. Although considerable discussion of the evolu- 
tionary significance of genetic drift has been generated, there has 
been little doubt that drift can occur, and thus it remains a factor 
to be reckoned with in all evolutionary studies. 

SUGGESTED READING 

Glass, B., 1954. "Genetic changes in human populations, especially those due to 
gene flow and genetic drift," Adv. in Genetics, 6:95-139. 

Li, C. C, 1955. Population genetics. Chicago: University of Chicago Press. 

Wright, S., 1951. "Fisher and Ford on the 'Sewall Wright effect'," Amer. Scientist, 
39/452-458. 



CHAPTER 



28 



The Origin of Subspecies 



New species can arise in two distinct ways, shown dia- 
grammatically below: 



d 

Time f I c 

b 



b i 

n T 



In I, only one species exists at any one point in time. Species a 
evolves into b, b into c, and so on; it is a "tran sformation in 
time." In II, a single species gives rise to two contemporary 
species; a splitting or "multiplication in space" has occurred, a 
process known as speciation, in a restricted sense of the word. 
Whereas the transformation of a single species in time is due to 
the combined effects of mutation, natural selection, and genetic 
drift, speciation involves an added problem: the origin, from a 
single species, of two or more species that no longer interbreed. 
Once established, they maintain their separate identities and pur- 
sue independent evolutionary paths. Our problem now is to con- 
sider the ways in which different populations of the same species 
with essentially the same genetic composition can diverge from 
each other. To do so, it is necessary to discuss. population struc- 
ture — that is, the way in which the individual members of a 
species are subdivided into breeding groups. 

Population Structure 

Some species may be common and widely distributed, 

268 



THE ORIGIN OF SUBSPECIES • 269 

forming one large, nearly continuous population over thousands of square miles 
of a continental land mass. The American robin (Turdus migratorius) and the 
red-winged blackbird (Agelaius phoeniceus) are species of this type. However, 
even though essentially continuous in their distribution, in that there are no 
gross barriers separating one segment of the species from the rest, nevertheless 
mating is not random over the entire species range, for obviously one male is not 
equally likely to mate with all of the females in the species. The chances that a 
male in Massachusetts will mate with Michigan or Minnesota females are virtu- 
ally nil; they are isolated by distance. 

Other "species populations clearly have a discontinuous distribution. 
A species inhabiting a series of islands is perhaps the most clear-cut example of 
this type, but a comparable situation is found in species living in a series of iso- 
lated lakes or marshes, in clumps of trees surrounded by prairie, on a particular 
type of soil, or only above a certain elevation in a mountain range. In each case 
each population is quite clearly delimited from the other populations of the 
same species by a zone in which no members of that species live. 

A variety of other population structures can be visualized, but we shall 
mention just one more, the linear distribution such as might be found in a 
species living in or along a river. A similar structure is found in species living 
along the seashore or at a limited elevation along a long mountain ridge. Here, 
the distribution is continuous, but again isolation by distance may be a modifying 
aspect. 

The distribution pattern of a species is determined by a number of 
factors, any one of which may act as ^baxriex-Mlurther expansion of the species' 
range. The barrier may be some obvious physical feature such as an ocean, a 
desert, or a mountain range. However, since an impassable barrier for one species 
may serve as a broad highway for another, even barriers that seem obvious cannot 
be so termed without reference to the kinds of organisms unable to surmount 
them. Consider, for example, the different role the ocean has played in the dis- 
tribution of whales and elephants. Climate, especially as related to temperature 
and moisture, may set limits on the range of a species, and such limits are quite 
as rigorous in their way as are the physical barriers. Furthermore, some plants are 
restricted by their soil, or edaphic, requirements to only limited portions of an 
otherwise suitable habitat. 

The ecological conditions, which are of course in part determined by 
the physical conditions, may also influence the distribution pattern of a species 
and^serve as a barrier to its expansion. One has but to think of species typical 
only of the prairie, or of coniferous forest, or of deciduous forest to realize that 
distribution also depends on the type of habitat available. Destruction of its 
habitat means the elimination of a species from that area. For this reason, 
present game and fish management practices are placing increasing emphasis on 
habitat improvement. These habitat needs may be both general and also quite 



270 • THE MECHANISM OF EVOLUTION 

specific. The distribution, for example, of oak-gall wasps of the genus Cynips 
was shown by Kinsey (who later became better known for other research) to be 
dependent on the distribution of the oak trees in which they laid their eggs. The 
yellow-headed and red-winged blackbirds are closely related species, both of 
which breed in Minnesota in cattail marshes. While the red-wing is found in 
almost every cattail marsh available, the yellow-head seems to breed only in those 
marshes where no willows or other shrubs or bushes encroach on the edges of 
the marsh. It is not surprising, therefore, that it is known as a bird of the 
prairies. 

For genetic divergence to take place within a species, it is essential that 
the original species population be divided into populations that are physically 
isolated from each other. Jf they are not isolated, interbreeding will occur and 
no divergence will be possible, for the species will be sharing a common gene 
pool, and continual hybridization will swamp any differences that might arise. 
The actual distances may be very great or quite small, depending on the species. 
A few hundred yards of unsuitable habitat may be quite sufficient to separate two 
snail populations, while several hundred miles' separation may be necessary to 
achieve the same degree of isolation in birds. The essential factor is not the 
absolute distance, but the lack of opportunity for mating between members of 
the different populations because of their separation in space. Some biologists 
have argued that ecological divergence could occur without physical isolation. 
However, the initial and crucial steps leading to divergence in ecological require- 
ments would be the most difficult and would be likely to occur only under the 
most favorable circumstances, if at all. 

At this point it may be worthwhile to review some of the terms used to 
describe the variability of natural populations. A breeding population or Men- 
delian population is a group of individuals tied together by bonds of mating and 
parentage and thus sharing a common gene pool. Since these individuals are not 
of a uniform genotype but are typically variable, the population is polymorphic. 
A species is polytypic if composed of genetically distinct breeding populations. 
Individuals living close enough to one another so that interbreeding between 
them is possible are said to be sympatric (that is, living in the same country). 
Those living at greater distances are allopatric. Thus polymorphic variability 
should be found in sympatric. individuals; if the variations are found only in 
allopatric populations, they are polytypic. 

Races or subspecies are biological units below the species level. They 
are geographically defined aggregates of breeding populations that differ from 
one another in the frequencies of one or more genetically determined traits. The 
definition of race or subspecies is rather fuzzy because the concept of race is 
itself rather fuzzy. For example, it is impossible to say, without being arbitrary, 
just how different two populations must be to warrant subspecific rank. Further- 
more, in some cases the traits of a species seem to change rather gradually across 



THE ORIGIN OF SUBSPECIES • 271 

the range of the species and a dine is said to exist. These gradual, continuous 
changes are the result of adaptation to similar gradual changes in such things as 
annual temperatures or rainfall. The difficulty in denning a race increases in 
species where clines are found, for even though the terminal populations may be 
quite different, if no sharp discontinuity exists, it is extremely difficult to delimit 
racial boundaries. Therefore, the concept is of limited usefulness and should be 
applied with caution. To dignify all infraspecific variation with subspecifk 
taxonomic names may serve only to compound confusion rather than to clarify it. 
In certain circumstances the labels may be of sufficient usefulness to justify using 
them, but the underlying biological situation should be kept clearly in mind. 

Isolation and Subspeciation 

The brief discussion of population structure above should serve to indi- 
cate that a species population usually has a discontinuous distribution. If its 
range is very large, even a more or less continuously distributed species does not 
form one large randomly mating population, simply because of the distances in- 
volved. Therefore, as a general rule, a species is composed of a number of allo- 
patricbreeding populations, each physically separated to some extent from the 
others and pursuing its own independent evolutionary path. Even though the 
genetic composition of these populations may initially be very similar, no two 
environments are likely to be biologically or physically identical, and thus the 
selection pressures on these populations will almost inevitably be somewhat dif- 
ferent. Selection plus the random aspects of mutation and, in small populations, 
of inbreeding and genetic drift will bring about divergence in the hereditary 
characteristics of the formerly similar populations. For this reason, it is to be 
expected that most widely distributed species will show variation among the dif- 
ferent breeding populations in different parts of the range. These differences 
may take the form of clines, or, when the variation is sufficiently well defined, 
different geographic races or subspecies may be recognized. 

A somewhat different mode of origin for genetic diversity between 
populations, suggested by Mayr, is known as the "founder principle." Although 
it does not involve any new concepts, the known principles are thought to 
operate in a somewhat different way from the usual method outlined above. In 
brief, the suggestion is ...that l if, for example, a small population colonizes a pre- 
viously uninhabited island, the gene pool introduced into the island may differ 
somewhat from that of the species as a whole. As a result, the selective value of 
the^enes may be somewhat different from their value in the parental population, 
hecause of their new genetic environment as well as the new external environ- 
ment. Thus, drift and selection pressures are thought to account for the some- 
times striking differences between different island populations and between 
island populations and their continental ancestors. 



272 • THE MECHANISM OF EVOLUTION 

In order to gain better insight into the nature of the differences between 
geographically isolated populations, let us consider a few selected cases that have 
been studied rather carefully. The coast tarweed, Hemizonia angustifolia, is a 
member of the sunflower family and is found in a narrow belt along the sea 
coast of California. Of the two races, one extends 275 miles along the coast from 
northern California to south of Monterey Bay; the other, after a gap of 40 miles 
of unsuitable habitat due to the Santa Lucia Mountains, ranges another 40 miles 
southward. Although the two races are geographically isolated from each other, 
they occupy ecologically similar habitats. Nevertheless, because there are small 
but consistent and significant morphological differences between them, they have, 
sometimes been called distinct species. Plants of the northern race have a low, 
broad habit, slender open branching, and rather small flower heads. The plants 
from the southern race have more erect, robust branching, and larger flower 
heads. The two races cross easily and produce fertile ¥ t hybrids. The F 2 showed 
that the slight differences between the races were due to numerous multiple 
factors. Of 1152 F 2 plants reared, no two were alike and no plant was exactly 
like either of the parents. Almost all possible recombinations of the parental 
traits were found. Whereas 57 percent of the F 2 individuals were as large as the 
parents, 43 percent were smaller in size, some being as much as 1000 times 
smaller than other F 2 plants (Fig. 28-1). Thus the genes in these two races have 
diverged sufficiently so that in some combinations they do not support develop- 
ment to normal size even though the combinations are viable. However, fertility 
and viability in the hybrids are sufficiently good to warrant calling these two 
groups geographical subspecies rather than separate species. Since both occupy 
the coastal plain, Clausen, Keck, and Hiesey, who made this study, consider 
them to form a single ecotype but two geographic races. To what extent the 
differences between them may be adaptive and to what extent they are of chance 
origin has not been determined. 

Genetic Differences between Subspecies 

A quite different situation has been described in the climatic or alti- 
tudinal races of the cinquefoil, Potentilla glandulosa, a member of the rose 
family. This species occurs in central California from the lowlands near the 
coast up to heights of 11,000 feet in the Sierra Nevada. At least seven climatic 
races have been identified. The extreme types, the lowland and the alpine races, 
are strikingly different both morphologically and physiologically. The lowland 
race grows throughout the year, but the alpine race is winter dormant for nine 
months. The alpine race is dwarf as are many alpine plants, but it has large 
flowers; the lowland plants, though large and robust, have small flowers. Trans- 
plantation experiments showed that alpine plants remained winter dormant for 
two or three months, even in the lowland environment, and grew rather poorly. 



THE ORIGIN OF SUBSPECIES • 273 










Classes 




320 105 

Total- 1152 F 2 plants 



Fig. 28-1. Genetic divergence between two geographical races of the coast tarweed, 
Hemizonia angustifolia. Top, left, the northern race (Pi); right, the southern race 
(P 2 ). F 2 , top, three vigorous, and bottom, three dwarf segregants. The scale beside 
each plant is 10 cm high. The cubes represent F 2 size classes, and the numerals 
below, the number of plants in each class. The cube to the left, 50 cm to a side, 
is comparable to the parents. The others are 35, 25, 15, 10 and 5 cm respectively. 
(Courtesy of Clausen, Keck, and Hiesey.) 



274 • THE MECHANISM OF EVOLUTION 

The Coast Range plants failed to survive the harsh winter at the alpine station. 
These transplantation experiments and others showed that even though the 
phenotype was modified to some extent by the environment in which the plant 
was raised, the fundamental differences between these races were genotypic and 
adaptive to the particular environment from which the plants came. The genetic 
basis for the morphological and physiological differences between these races 
was confirmed by the results from crosses among them. Since the hybrids were 
all vigorous and fertile, no reproductive barrier exists among the various races. 
In the F 2 generation, genetic recombination resulted in a complete reshuffling of 
the parental traits. Some of the new F 2 combinations showed some rather surpris- 
ing abilities. For example, some were more vigorous and frost resistant in the 
alpine habitat than the native alpines. Many that were well adapted to the alpine 
climate had vegetative characteristics of the parents from the lower elevations. 
Some thrived at all elevations from sea level to the alpine station, unlike any of 
the parent races. One recombinant type appeared as though it might be well 
adapted to the extreme maritime environment, which this species has not yet 
been able to invade successfully. The races were distinguished from one another 
by a dozen or more easily recognizable traits. Segregation and recombination in 
the F 2 showed that these differences were governed by multiple factors rather 
than single gene differences. The results from all of these experiments indicate 
that the differences between these races are adaptive and have evolved gradually 
through the accumulation of numerous small genetic differences. Furthermore, 
the potentialities for further evolution may be greatly enhanced by the release of 
variability brought about by hybridization between subspecies. 

In the leopard frog, Rana pipiens, a somewhat similar but in certain re- 
spects quite different situation exists. This species ranges from northern Canada 
far down into Central America. As might be expected, individuals from different 
geographical areas show morphological differences, and on these grounds a 
number of subspecies have been named. However, no general agreement about 
the subspecies has been reached, for the characters used are not reliable and the 
continuous distribution of this species makes lines of demarcation difficult to draw. 
Moore has shown that the leopard frog is able to exist in this wide range of 
environments because the southern populations of Rana pipiens differ in adaptive 
traits from the northern populations in much the same way that southern species 
of frogs differ from northern species. Thus, for example, in temperature toler- 
ance and rate of development, the northern frogs were able to tolerate and 
develop normally at lower temperatures than southern frogs, but could not tol- 
erate the higher temperatures at which southern frogs still developed normally 
(Fig. 28-2). Data on other traits gave comparable results, suggesting that these 
populations, too, have become genetically adapted to their environments. How- 
ever, unlike the crosses between races of Potentilla glandulosa, which gave 
normal, fertile hybrids, crosses between frogs of northern and southern origin 



THE ORIGIN OF SUBSPECIES • 275 




TEMPERATURE RANGE FOR 
NORMAL DEVELOPMENT 



Fig. 28-2. Geographic variation in embryonic temperature 

tolerance in Rana pipiens. Upper and lower limits are given 

in degrees C. A question mark indicates lack of data. (With 

permission of Moore.) 

gave rise to inviable hybrids. Thus the extreme populations behave as good 
species toward each other. However, since adjacent populations are fully inter- 
fertile, no barrier to genetic exchange exists throughout the range of the species, 
and it is best treated as a single species in which divergent populations have 
arisen owing to adaptation to local environmental conditions, particularly with 
respect to temperature. 



► SUMMARY 



A study of the population structure of a species typically 
reveals that it is composed of a number of more or less isolated 
breeding populations. Since the habitat is unlikely to be uniformly 



276 • THE MECHANISM OF EVOLUTION 

favorable throughout the species' range, this structure is to be 
expected. The origin of genetically divergent groups or subspecies 
within a species virtually requires some degree of isolation be- 
tween breeding populations, for otherwise, any differences that 
might arise would be swamped by hybridization. This isolation 
should not be thought of in terms of any absolute distance be- 
tween populations, but rather as the lack of opportunity for 
mating between the members of different groups. Since conditions 
are seldom, if ever, completely identical, the differing selection 
pressures, plus the random effects of mutation and genetic drift, 
tend to bring about genetic divergence between the different 
populations. The result is the formation of populations especially 
well adapted to their conditions of existence and differing from 
other populations of the same species living under somewhat 
different environmental conditions. Although the possibility of the 
sympatric differentiation of one population into two distinct 
breeding populations cannot be completely excluded, it must, in 
view of the difficulties attendant on such an event, have played 
only a minor role in the evolutionary process. The establishment 
of genetic differences between different breeding populations of 
the same species is the first step toward the origin of species. 

SUGGESTED READING 

Clausen, J., and W. M. Hiesey, 1958. "Experimental studies on the nature of species. 
IV, Genetic structure of ecological races," Carnegie Institute, Washington, 
D. C, Publ. 615. 

Mayr, E., 1942. Systematic s and the origin of species. New York: Columbia Uni- 
versity Press. 

, 1959- "Isolation as an evolutionary factor," Proc. Amer. Philosophical 

Society, 103(2) :221-230. 

Moore, J. A., 1949. "Geographic variation of adaptive characters in Rana pipiens 
Schreber," Evolution, 3.T-24. 



CHAPTER 



29 



Hybridization and Evolution 



We have just considered the role of isolation in the 
origin of subspecies, and we must now consider what happens if 
for some reason isolation breaks down and interbreeding again 
occurs between formerly isolated and divergent populations. The 
importance of hybridization to evolution has been overstressed by 
some, who think there is a hybrid under every bush and often 
that the bush is a hybrid, too. Others have dismissed it as of no 
significance. The truth probably lies somewhere between these 
extremes, with hybridization more important to plant than to ani- 
mal evolution. In plants there is no psychological isolation, sexual 
reproduction is more efficient than in animals, and the individuals 
are longer lived — all factors that contribute to successful hybridi- 
zation. However, hybrids in animals have been identified in 
natural populations of fresh-water fishes, toads, and warblers, 
proof that hybridization does occur in animals as well as in plants. 

The breakdown of isolation may come about in a variety 
of ways. Physical changes in the environment due to fires, floods, 
earthquakes, volcanic eruptions, or other catastrophes may drasti- 
cally alter the habitat. Changes in climate, and the resultant 
changes in precipitation, the retreat of glaciers, land-bridge for- 
mation, all may lead to renewed contact between formerly isolated 
groups. The environment does not remain stable indefinitely, but 
undergoes both local and regional shifts in character in many 
ways. 

Changes in the biota may also radically alter the environ- 
ment. The goats introduced on Pitcairn Island have kept the 



277 



278 • THE MECHANISM OF EVOLUTION 

island virtually denuded of large trees. Of all the species, however, man 
has had the greatest impact on the environment all over the world. His 
activities — clearing forests, burning over land, planting crops, draining swamps, 
and building roads, railroads, dams, homes, towns, and cities — have dis- 
rupted the environment almost beyond recognition or belief in many in- 
stances. With him he has carried weedy species of plants and animals to 
all parts of the earth. The rabbit with its depredations on the range lands of 
Australia is a familiar example. The impact of such species as man and the 
rabbit is direct and obvious, but the interrelationships among organisms are so 
complex and interwoven that a single change, like a stone in a pond, may set in 
motion a chain of events in an ever-widening circle. The classical example of the 
effect of the number of spinsters on the red clover crop will serve to illustrate 
this point (Fig. 29-1). Clover depends for fertilization on the bumblebee; field 
mice feed on bumblebee nests; cats prey upon the mice; and it is well known 
that old maids keep cats for company. Thus, it is obvious that the larger the 
number of spinsters, the better the clover crop. 



The Effects of Migration 

The effects of hybridization will differ to some extent, depending on the 
degree of genetic divergence between the populations involved. Let us consider 
first the simple case in which the populations differ very little. Imagine a popula- 
tion of mice on an island a short distance off the mainland coast, from which 
migrants regularly reach the island. Assume that the frequency of the gene A is 
0.4 in the mainland population but only 0.2 on the island. The effect of these 
immigrants on the frequency of A in the island population will depend on their 
genetic contribution to the island population, which is measured by m, the co- 
efficient of replacement. The value of m is determined by the proportion of 
gametes contributed to the next generation by the immigrants. The change, due 
to immigration, in the frequency of A on the island is given by the equation, 

Ap = —mQp — pnO 
where p = frequency of A on the island 

p m = frequency of A among the immigrants 
m = coefficient of replacement 

If m is equal to 10 percent, then 

Ap = -0.1(0.2 - 0.4) = -F0.02 
po + Ap = pi = 0.20 + 0.02 = 0.22 



HYBRIDIZATION AND EVOLUTION • 279 



&r 




®jtm 




■j? \r 





Fig. 29-1. Biological complexity: the effect of spinsters on the red clover crop. 



When p = p m , an equilibrium will be established. If the above rate of immigra- 
tion persists, it is clear that an equilibrium will soon be reached and that the 
island population cannot retain its individuality. 

In many respects the effects of migration or gene flow are similar to 
those of mutation, for both mutation and migration introduce new genes into a 
population. By migration, favorable genes or gene combinations can spread 
throughout a species from the population in which they arose. Thus, migration 
tends to make local populations more nearly alike in gene frequencies and to 
prevent any significant local differentiation within a species. If isolation is com- 
plete (m = 0), each population will pursue an independent course. For values 
of m other than zero, the consequences of migration will depend on the relation- 



280 • THE MECHANISM OF EVOLUTION 

ship between the amount of gene flow and the factors such as selection pressure 
and genetic drift that operate within each breeding population. If, for example, 
the intensity of selection, as measured in terms of the selection coefficient (j), is 
greater than the effect of immigration as expressed by the coefficient of replace- 
ment, then local gene frequencies will depend largely on selection pressure, with 
migration having only a minor diluting effect. On the other hand, if m is greater 
than j, the gene frequencies in the local populations will not differ greatly from 
the average frequencies in the total population. 

Introgressive Hybridization 

When hybridization occurs between two subspecies or species, the ¥ t is 
usually quite uniform and intermediate in phenotype to the parents. If formed, 
the F 2 is quite variable, because of the recombination of numerous gene pairs. 
However, the rare, naturally occurring hybrids have a much greater chance of 
back crossing to one of the parent species than of mating with each other, and 
therefore a simple F 2 would only seldom be expected. Thus, where hybridization 
is taking place under relatively stable environmental conditions, three distinct 
groups, hybrids and the two parent species, are generally not found. Instead, the 
parent species will be somewhat more variable than in other areas where they are 
not sympatric, and each will show some traits suggestive of the other species. 
This type of situation is known as introgressive hybridization. The nearer a back- 
cross individual resembles one of the well-adapted parents, the better its chances 
of survival in a stable environment, and hence the more subtle the introgression 
of the foreign genes. However, if the hybrids are formed in a highly disrupted, 
unstable environment, new and different adaptive types may be formed that are 
better adapted to the new conditions than either of the parents. Thus gene flow 
may occur, even across partial interspecific barriers. An example of introgression 
has been found in the Mississippi delta country of Louisiana. Iris fulva grew in 
clay soil and partial shade while Iris hexagona, a closely related, but quite 
different-looking species, grew in full sunlight in the tidal marshes (see Fig. 
29-2). The clearing of the woodlands and the draining of the swamps have led 
to considerable introgression in these two species, in some cases with hybrid 
populations persisting to fill newly created ecological niches. Numerous other 
examples have been described in plants, most of them in areas disturbed by man. 
However, two species of sugar maples, ecologically distinct in southern Michigan, 
have been found hybridizing in a formerly glaciated part of Quebec. When it is 
realized that many parts of North America were covered by glaciers as recently 
as 10,000 to 12,000 years ago and that all the animals and plants now living in 
these areas must have reinvaded them not so very many generations ago, it is 
easier to visualize how rapidly conditions may change for a given species and 
how isolation may arise and then break down. 



HYBRIDIZATION AND EVOLUTION • 281 





Fig. 29-2. Introgression in iris. Below: Flowers and enlarged 
sepals of Iris fulva (left) and Iris hexagona var. giganti- 
caerulea (right) to the same scales. Above: Map of the area 
where these two species are hybridizing. H-l and H-2 are 
two somewhat different hybrid colonies. (With permission of 
Anderson. ) 



Polyploidy and Evolution 

Introgression is possible only if the hybrid is at least partially fertile. 
However, even if hybrid sterility blocks direct gene flow, genes from two dif- 
ferent species may still form viable, fertile polyploids. Most natural polyploids 
are the result of hybridization between two species, with a subsequent doubling 



282 • THE MECHANISM OF EVOLUTION 

in the number of chromosomes, and hence are allopolyploids. Even many poly- 
ploids thought originally to be autopolyploids derived from a single species have 
frequently, on closer study, been shown to be allopolyploids. 

In the broad sense of the word "mutation," polyploidy is a mutational 
change. It is the only known method by which cataclysmic evolution can occur, 
giving rise to a new species in a single step, for a new polyploid species is fertile 
and true breeding yet is reproductively isolated from both parent species. How- 
ever, it is a specialized and restricted form of evolution, occurring primarily in 
plants and involving the recombination of existing genes rather than the creation 
of anything truly new. 

Polyploids frequently have different distributions and different ecological 
preferences from their diploid relatives, and are generally thought to be more 
tolerant of extreme ecological conditions. For example, in Biscutella laevigata of 
the mustard family Cruciferae, the tetraploids have a continuous distribution over 
much of Europe including the Alps, the Carpathians, and the mountains of 
Italy and the northern Balkans. The diploids have a discontinuous distribution 
and are confined to the valleys of the Rhine, Elbe, Oder, and upper Danube. See 
Fig. 29-3. The diploids are confined to regions that were not covered by the ice 
sheets during the glacial period and hence were open to habitation for a long 
time. The tetraploids exist now in the areas formerly covered by the ice sheet 
and must have invaded these areas from elsewhere while the diploids were ap- 
parently unable to do so. The wider distribution of polyploids may be due to a 
wider range of adaptability, which permits them to invade and colonize areas 
newly open to plants. 

In several cases it has been possible to resynthesize naturally occurring 
polyploids and thus prove not only their hybrid origin but also their exact 
parentage. For instance, the mint Galeopsis tetrahit with In — 32 has been re- 
synthesized from G. pubescens and G. speciosa, each with In = 16. The syn- 
thetic polyploid is similar in morphology, cytology, and genetics to the natural 
species. 

In animals, polyploidy is rare and must, therefore, have played only a 
minor role in animal evolution. The few known animal polyploids occur almost 
exclusively in hermaphroditic or parthenogenetic species. Its rarity is very prob- 
ably due to the separation of the sexes in animals, for polyploidy almost in- 
evitably upsets the chromosomal sex-determining mechanism. The normal diploid 
female in most animal species has two sets of autosomes plus two X chromo- 
somes; the male has two sets of autosomes plus an X and a Y chromosome. In 
triploid or tetraploid individuals there may be an imbalance between the X and 
the Y chromosomes (XXY, XYY, XXXY, etc.) or between the sex chromo- 
somes and the autosomes, so that in most cases they are intersexes or sterile or 
otherwise abnormal. Under these circumstances, maintenance of a stable poly- 
ploid condition is very improbable. Since polyploid tissues have been observed 



HYBRIDIZATION AND EVOLUTION • 283 




TETRAPLOIDS 
• all Forms known to be letraploid . 
x forms not investigated but probably tetraploid. 
DIPLOIDS 
03 Sep. gracilis <t> Ssp alsatica O Ssp subaphytla 

A var mollis 
FORMS NOT INVESTIGATED BUT PROBABLY DIPLOID 



EB Ssp. iernen 



A Ssp. 3ustriaca 



, ( Ssp guestphahca 
{Ssp. teswifolia 



D rar paruifoha 



Fig. 29-3. Detailed distribution of diploid and tetraploid forms 

of the cruciferous plant Biscutella laevigata in Central Europe. 

(Adapted by Manton from Machatschki-Laurich.) The thick black 

lines represent the boundaries of the ice sheets. 



284 • THE MECHANISM OF EVOLUTION 

in diploid species, polyploidy is at least possible in animal cells; in fact, poly- 
ploid animals have occasionally been reported. Even in man a triploid has been 
found, and Klinefelter's syndrome, characterized by faulty development of the 
seminiferous tubules, has been shown to be an XXY intersex condition. Hence, 
the abnormal sexual development in animal polyploids appears to constitute a 
major barrier to their success. 

Since animal evolution has proceeded normally in many lines in which 
no polyploids are found, polyploidy cannot be an essential part of the evolu- 
tionary mechanism. On the other hand, at least one third of all species of higher 
plants are polyploid, an indication that polyploidy has obviously played a major 
role in plant evolution. Nevertheless, it has been suggested that major evolu- 
tionary advances have been confined to the diploid lines even in plants, and that 
polyploids may lead to evolutionary dead ends because of their greater pheno- 
typic stability. 

Evolutionary changes involving major adaptive shifts typically occur at 
exceptionally rapid rates under changing environmental conditions. Mutation 
rates are thought to be generally too low to provide at any one time the vari- 
ability necessary to permit such rapid rates of evolution. However, the primary 
effect of hybridization between members of different populations is to increase 
greatly the available genetic variability through genetic recombination. There- 
fore, hybridization has been hypothesized as being especially favorable to rapid 
rates of evolution. If this is the case, then hybridization has a peculiarly signifi- 
cant role in the evolutionary process. Furthermore, the familiar phylogenetic 
diagram in the form of a branching tree is incomplete, for the pattern should be 
reticulate as well as branching. 



SUMMARY < 



Hybridization between members of different breeding 
populations may result from a breakdown in isolation between the 
groups. The consequences of hybridization depend upon a num- 
ber of circumstances. If the populations are genetically rather 
similar, hybridization may be treated as migration or gene flow 
from one population to the other, which will tend to reduce and 
eventually eliminate the genetic differences between them. Thus, 
extensive gene flow tends to prevent local differentiation of popu- 
lations within a species. Hybridization between species or rela- 
tively well-defined subspecies may lead to introgressive hybridiza- 
tion, the introduction of some genes from one population into the 
other. An increase in genetic variability may thus occur without 
a complete swamping of the identity of the parental populations 
by hybridization. In plants, hybridization followed by chromo- 
some doubling has frequently resulted in the cataclysmic origin 
of new polyploid species, reproductively isolated from their 
parents. 



HYBRIDIZATION AND EVOLUTION • 285 

SUGGESTED READING 

Anderson, E., 1949. Introgressive hybridization. New York: Wiley. 

Stebbins, G. L., 1950. Variation and evolution in plants. New York: Columbia Uni- 
versity Press. 

, 1959. "The role of hybridization in evolution," Proc. Amer. Philosophical 

Society, 103(2) :23 1-251. 



CHAPTER 



30 

Isolating Mechanisms 



In the two previous chapters we discussed the causes of 
genetic divergence between allopatric populations and the effects 
of hybridization on such populations if they again become sym- 
patric. However, during periods of isolation, populations may 
diverge to the point where they do not interbreed even when 
they become sympatric again. This reproductive isolation is due to 
the development of various isolating mechanisms, which serve to 
prevent or reduce the amount of interbreeding. Geographicaljor 
spatial isolation effectively prevents gene, exchange only -so long 
as it exists, but isolating mechanisms are under genetic control 
and will maintain reproductive isolation even between populations 
that again come in contact with one^ another. Virtually all of the 
evidence suggests that the initial stages in the development of 
isolating mechanisms must occur during a period of spatial, isola- 
tion. Therefore, the changes leading to reproductive isolation 
must be incidental to the genetic divergence that occurs during a 
period of isolation. Crossing between members of closely related 
groups may be prevented in a variety of different ways, of which 
we shall consider several for purposes of illustration. 

Types of Isolating Mechanisms 

Ecological isolating mechanisms are quite common. In 
the deermouse, Peromyscus maniculatus, two races inhabit imme- 
diately contiguous areas in Michigan but nevertheless retain their 
identities. One race is confined to the sandy lakeshore beaches 

286 



ISOLATING MECHANISMS • 287 

wiiile the other inhabits the forest that starts just a short distance back 
from the shore. Their habitat preferences are evidently so well denned 
that interbreeding is negligible. The white crappie and the black crappie 
(fresh-water fish) inhabit the same streams in Indiana, but despite similar 
food and other habits, they seldom interbreed, for the white crappie is 
active by day and the black at night. Edaphic, or soil, conditions isolate the 
spiderwort, Tradescantia canaliculata, which grows in full sunlight at the tops of 
cliffs, from T. subaspera, which grows in the shade at the bottom. Given the 
opportunity, these two species hybridize readily. 

Seasonal isolation may be a very effective barrier to gene exchange. In 
cockleburs, for example, the flowering times of two species have become so dif- 
ferent that in the same area one species flowers only after the other has formed 
its seed capsules, and the chances of crossing are nonexistent. The American toad 
(Bufo americanus) and Fowler's toad (B. fowleri) have quite similar distribu- 
tions and form fully fertile and viable hybrids in laboratory crosses. However, 
the two species remain distinct because B. americanus breeds early in the season 
whereas. B. fowled breeds late. The occasional hybrids between the species are 
Tound in situations where tKeHhabitat has been disturbed, indicating a difference 
in ecological requirements of the species as well. 

The most complex behavior patterns in animals are generally in some 
way associated with,-j£production. In essence, courtship consists of a series of 
stimuli and responses between male and female, with each response serving as a 
new stimulus. It apparently functions primarily to arouse readiness for mating 
and to synchronize mating behavior rather than to influence the choice of mates. 
However, jf a male starts to court a female of an entirely different species, the 
courtship is usually broken off rather quickly because their behavior patterns do 
not mesh.. In this sense, courtship does restrict the choice of mates. This type of 
isolating mechanism is usually referred to as sexual isolation and is based on 
"psychological" or ethological differences. The lack of mutual attraction has been 
traced to differences in scents, behavior patterns, sexual recognition signs, and 
similar traits. Ethological isolation generally precedes the development of sterility 
barriers and thus is one of the first isolating mechanisms to appear. American 
ducks such as the mallard and the black duck cross readily in captivity and pro- 
duce fully fertile offspring, but hybrids in nature are very rare. The eastern 
meadowlark {Sturnella magna) and the western meadowlark (S. neglecta) are 
much alike in appearance and have broadly overlapping ranges but nevertheless 
seldom interbreed in the zone of overlap. In both cases sexual isolation must play 
a major role in their reproductive isolation even though other factors undoubt- 
edly contribute also. Although not a factor in plant evolution, the evolution of 
behavior patterns is of great interest to zoologists, and comparative ethology has 
been a rapidly growing field of study. 

Another group of phenomena may be called physiological isolating 



288 • THE MECHANISM OF EVOLUTION 

mechanisms. For example, the sperm of Drosophila virilis males show a lower 
viability in the reproductive tract of alien females (D. americand) than„irjLiffijT 
own females. After copulation in some species of Drosophila, the vagina swells 
greatly owing to the secretion of fluid into the cavity. This insemination reaction 
is accentuated to such an extent in interspecific crosses that fertilization and egg 
laying may both be blocked for days. In plants, the growth rate of the pollen 
tube may be slower than normal on a foreign style or in some cases the pollen 
tube may even burst. Physiological barriers of this sort serve to limit or prevent 
the union of the gametes so that fertilization does not occur. 

Even if fertilization between gametes from different populations takes 
place, hybrid inviability may intervene to prevent the development of a viable 
hybrid organism. The zygote may cease development at almost any stage, early 
or late, or may develop into a grossly deformed monster. Such a situation even 
exists within a single species, Rana pipiens, in which hybrids from crosses be- 
tween leopard frogs from northern and southern United States are deformed and 
inviable. The inviability of the hybrids results from a disharmony within the 
embryo, preventing normal development. In plants, another type of disharmony, 
between the hybrid embryo and the seed coat, a maternal tissue, sometimes blocks 
normal growth. This effect can be circumvented by removing the embryo from 
the seed and culturing it in vitro. Embryo culture has been used to rear several 
plant hybrids that had never before been successfully grown. 

Interspecific crosses occasionally result in progeny that are all of the 
same sex. Hybrid inviability is thus confined to just one of the sexes. Haldane 
perceived that when one sex is absent or rare or sterile in such F x hybrids, then 
that sex is the heterogametic sex. This generalization is sometimes known as 
Haldane's rule. Accordingly, the male hybrids are defective in most species 
crosses except in birds, moths, and butterflies, the groups in which the females 
have ZW sex chromosomes and hence are the heterogametic sex. 

In many cases, normal, vigorous hybrids are formed, but are sterile. The 
further exchange of genes is in this way completely blocked. The mule is .the 
classical example of hybrid sterility. Any one of a number of conditions may 
cause hybrids to be sterile. In general, either the sex organs fail to develop suffi- 
ciently for meiosis to take place, or else abnormalities in the meiotic process itself 
(for example, in synapsis or spindle formation) prevent the formation of normal 
gametes. 

Even when vigorous, fertile F t hybrids are produced, hybrid breakdown 
in the F 2 or back-cross generations may contribute to reproductive isolation. In 
such instances the subsequent generations may manifest reduced vigor or fertility 
or both. 

All of the isolating mechanisms mentioned above are in some way 
genetically controlled and will restrict the exchange of genes between different 
groups of animals or plants. Once reproductive isolation of this sort is firmly 



ISOLATING MECHANISMS • 289 

established, the evolutionary paths of these groups will have passed the point of 
no return. No longer will they combine to form a common breeding population. 
Generally, several isolating mechanisms exist between different species; thus, 
even though no one mechanism is completely effective, their combined effects 
cause total reproductive isolation. A major problem is to account for their mode 
of origin, for the achievement of reproductive isolation is the crucial step in 
speciation. 

The Origin of Isolating Mechanisms 

Two major theories have been proposed to explain the origin of iso- 
lating mechanisms. Muller suggested that reproductive isolation is an incidental 
by-product of the genetic divergence that occurs during the origin of subspecies 
and species in allopatric populations. In other words, as the evolving populations 
adapt to their different environments, a reshuffling and restructuring of the 
genes, the chromosomes, and the entire genotype occurs. As a result, if the popu- 
lations again become sympatric, incompatibilities causing reproductive isolation 
will already exist. Dobzhansky's theory is that reproductive isolation arises as a 
result of natural selection. He, too, recognized the genotypes as integrated sys- 
tems of genes that, when drawn from different populations, may be incompatible. 
Hybrids often are poorly adapted or partially sterile and hence they will tend to 
be eliminated by natural selection. Since selection eliminates not only the hy- 
brids but at the same time the genes of the parents that hybridized, selection is 
acting against hybridization itself. Those individuals that hybridize and those 
genes favoring hybridization will gradually be eliminated from the population. 
Natural selection thus acts to reduce the wastage of gametes on the less-fit hy- 
brids. These theories, of course, are not mutually exclusive but complementary. 
Although some relevant evidence is available, additional research is needed to 
evaluate the relative importance of these two mechanisms and to clarify 
still further the basis for the reproductive isolation between closely related 
populations. 



Isolating mechanisms, which are mechanisms for main- 
taining reproductive isolation between sympatric populations, are 
under "genetic* control. They may be ecological, seasonal, etho- 
logical, or physiological barriers to fertilization; or, if fertilization 
occurs, hybrid invi ability, hybrid sterility, or hybrid breakdown in 
theJF^jimy.. intervene to restrict the successful exchange of genes 
between different populations. Isolating mechanisms have been 
thought to arise as an incidental by-product of the genetic diver- 



► SUMMARY 



290 • THE MECHANISM OF EVOLUTION 

gence occurring during speciation, but it has also been postulated 
that natural selection against poorly adapted hybrids — in the final 
analysis, selection against hybridization itself — will tend to build 
up barriers to crossing. 

SUGGESTED READING 

Dobzhansky, Th., 1951. Genetics and the origin of species, 3d ed. New York: 
Columbia University Press. 

Mayr, E., 1942. Systematic s and the origin of species, New York: Columbia Univer- 
sity Press. 

Stebbins, G. L., 1950. Variation and evolution in plants. New York: Columbia Uni- 
versity Press. 



CHAPTER 



31 



The Origin of Species 



Up to this point we have used the word "species" with- 
out defining the term. This vagueness has been purposeful. Now, 
however, as we begin our discussion of the origin of species, a 
definition is clearly in order. One reason for having avoided the 
question until now is that so many definitions exist. For example, 
a serological species definition runs like this: 

A species of helminths may be tentatively defined as a 
group of organisms, the lipid-free antigen of which, when diluted 
1:4,000 or more, yields a positive precipitin test within one hour 
with a rabbit antiserum produced by injecting 40 mg of dry weight, 
lipid-free antigenic material and withdrawn 10 to 12 days after the 
last of 4 intravenous injections every third day. 

This definition certainly has precision, though just what 
it signifies is a little less obvious. 

Another type of definition: "A species is what a compe- 
tent taxonomist considers to be a species." The problem is now 
simplified. Rather than classifying organisms, we now classify 
taxonomists into two categories: A. Competent; B. Incompetent. 

A definition rather surprising in that it came from a 
geneticist is, "Distinct species must be separable on the basis of 
ordinary preserved material. This is in order to make it possible 
for a museum man to apply a name to his material." This state- 
ment is an extreme form of a whole group of definitions that use 
morphological criteria to distinguish between species. Further- 
more, it suggests that the primary purpose of taxonomy is to 
facilitate the handling of museum specimens. 



291 



292 • THE MECHANISM OF EVOLUTION 

The Species as a Biological Unit 

Another group of definitions has come to be known as biological species 
definitions in contrast to the morphological definitions. Mayr has said, "Species 
are groups of actually or potentially interbreeding natural populations that are 
reproductively isolated from other such groups." Dobzhansky wrote, "Species are 
formed when a once actually or potentially interbreeding array of Mendelian 
populations becomes segregated in two or more reproductively isolated arrays," 
or, more briefly, "A species is the most inclusive Mendelian population." These 
definitions treat the species as a dynamic unit, a stage in the process of evolution, 
and not as a fixed static entity. The emphasis lies on the achievement of repro- 
ductive isolation, with the critical point in the origin of species the fixation of 
discontinuity between different populations. At the point when genetic discon- 
tinuity has been reached so that two populations thenceforward pursue inde- 
pendent evolutionary paths, species status is attained; up to that point they must 
be regarded as races or subspecies. The morphological species definitions are 
subjective, for they depend on the judgment of the taxonomist as to the degree 
of morphological similarity or difference worthy of species status. The biological 
definitions are more objective, for the behavior of the organisms themselves is 
the factor that determines their relationship. The significant question is whether 
they do or do not interbreed. The question is not whether they can interbreed 
but whether they actually do. Under experimental conditions, many "good" 
species can be induced to cross and may produce viable, fertile offspring; but if, 
under natural conditions, little or no gene flow occurs between them, their evolu- 
tionary paths remain separate and distinct. Most North American ducks, for 
example, are completely interfertile, but hybrids are extremely rare, and so they 
remain distinct species. 

The primary objective of a species definition is to describe as well as 
possible the natural biological relationships of the populations involved. The 
species is a natural biological unit tied together by bonds of mating and sharing 
a common gene pool. For this reason, it has objective reality. All of the other 
taxonomic categories — subspecies, genera, families, orders, etc. — are the subjec- 
tive creations of taxonomists, for the criterion of reproductive isolation is in- 
applicable. Among the various taxonomic categories the species is unique. 

Although the biological definition comes closest to describing the bio- 
logical realities, its application may lead to difficulties. For one, the definition is 
essentially nondimensional, not applicable to species living in different places or 
at different times, for it cannot be determined whether populations isolated from 
each other either in space or in time will actually interbreed. To the museum 
taxonomist working with dead specimens, and especially to the paleontologist, 
who has no choice but to work with nonliving materials, this definition is of no 
value. However, again we must return to the objectives of a species definition. 



THE ORIGIN OF SPECIES • 293 

Undoubtedly, if it were possible, paleontologists would prefer to use a biological 
criterion; it would probably considerably simplify the nomenclature in some 
groups. And modern taxonomy is rapidly moving beyond the point of relying 
solely on morphological traits in dead specimens, but is utilizing information on 
all aspects of the biology of a group in arriving at valid taxonomic groupings. 
A further implication of the biological definition is that two morphologically 
similar groups may be distinct species while two groups widely divergent in 
morphology may belong to the same species. The reasons for this situation are 
relatively simple. The morphology of an organism is essentially a reflection of 
its physiology, and physiological changes leading to reproductive isolation may 
well precede any major morphological changes. On the other hand, adaptive 
shifts leading to morphological changes may not affect the basic reproductive 
pattern sufficiently to lead to reproductive isolation. These possibilities are not 
merely theoretical; certain reproductively isolated species of Drosophila show 
virtually no major morphological differences. Drosophila pseudoobscura and D. 
persimilis, for instance, were formerly known as races A and B of D. pseudo- 
obscura. In contrast, European and American sycamores of the genus Platanus 
are quite different in appearance and have been assigned specific rank (P. 
orientalis and P. occidentalism ; yet their interfertility when grown together indi- 
cates that species distinction may be unwarranted. One final difficulty with the 
biological species definition is that it is limited to sexually reproducing species. 
In groups reproducing asexually, evolutionary change can occur only by se- 
quential mutations in a given line, with selection between lines. Since each line 
of descent is isolated from the others, each is pursuing an independent evolu- 
tionary path, but this hardly justifies assigning specific rank to each. Sexual 
reproduction is practically universal among the more complex or highly evolved 
animals and plants, very probably because evolution can proceed more rapidly in 
sexually reproducing species. Genes and gene combinations favored by selection 
can be combined and recombined in a manner impossible with asexual repro- 
duction, and hence adaptation and evolution are more flexible and more rapid. 
However, in spite of the difficulties inherent in the biological species definition, 
the morphological species and the biological species generally agree, and the 
exceptional cases are most instructive. 

Modes of Evolution 

The ways in which species originate are two or possibly three. Specia- 
tion, or the multiplication of species, leads to an increase in the number of con- 
temporary species. All the basic problems of evolution are wrapped up in the 
process of speciation, the way in which one species can split into two, and to 
this question we have devoted most of our attention. In brief, two or more 



294 • THE MECHANISM OF EVOLUTION 

populations of a species, upon becoming physically isolated, may diverge as the 
result of different mutation pressures, selection pressures, random genetic drift, 
or the net effect of all three. If gene flow is still possible through hybridization, 
migration pressures will be exerted, with the more favorable genes or gene com- 
binations being disseminated throughout the species. In this fashion a complex 
evolutionary pattern may develop, involving interpopulation selection. However, 
if the isolated populations diverge to the point of reproductive isolation, they 
will have achieved the status of distinct species. The process of speciation is 
diagramed in Fig. 31-1. 

The transformation of a species in time is a second mode of evolution. 
Simpson recognizes two types of transformation, which he has called "phyletic" 
evolution and "quantum" evolution. Phyletic evolution involves a sustained, 
directional shift in the average characters of a population; it is, in other words, 



Time 





Adaptive zone 

Fig. 31-1. Speciation: an increase in the number of species, 

achieved when the different populations become reproductively 

isolated. (After Simpson.) 



a line of succession rather than an increase in the total number of existing 
species. Phyletic evolution may be due to adaptation to a shifting environment or 
to increasing specialization or improved adaptation in a constant environment, 
and may be thought of as leading eventually to the origin of new genera and 
families. Diagrammatically, phyletic evolution is shown in Fig. 31-2. Most 
paleontology is devoted to the study of phyletic evolutionary changes. 

Quantum evolution, also known as mega- and macroevolution, is the 
term applied to the rapid shift of a population to a new equilibrium distinctly 
unlike the ancestral condition, thus leading to the origin of higher taxonomic 
categories such as new orders and classes. The origin of the higher taxonomic 



THE ORIGIN OF SPECIES • 295 

categories has presented a problem because new orders and classes generally ap- 
pear suddenly in the fossil record, without evidence of intermediate fossil types. 
If evolution is a gradual process, as both Darwin and modern theory hold, then 
it might be expected that fossils connecting different orders would be found as 
evidence of the gradual evolutionary transition from one group to another. Their 
absence has led some students of evolution to postulate that a different mechan- 
ism is responsible for the origin of higher groups, and that mutation, selection, 
gene flow, and genetic drift are responsible only for microevolutionary changes. 
Macroevolution has, for instance, been attributed to extremely rare macro- 
mutations or systemic mutations, which have such drastic effects that they give 
rise to "hopeful monsters." If, perchance, a "monster" is adapted to a new and 
different way of life, then the new adaptive type survives, and because it is so 
different, it clearly belongs in a new taxonomic group. For example, the Diptera, 
or two-winged flies, are clearly derived from the four-winged insects, with the 




Adaptive zone 

Fig. 31-2. Phyletic evolution: transformation in time leading to origin of new 
genera and families. (After Simpson.) 



gyroscopic halteres homologous to the second pair of wings. Since a mutation, 
tetraptera, is known that converts a dipteran into a four-winged insect, thus at 
one step excluding it from its own order, it is quite conceivable that at some time 
in the past the reverse occurred and the Diptera were derived from some four- 
winged insect order by a single systemic mutation giving rise at one step to a 
two-winged insect and hence to a new order. However, such an origin for higher 
taxonomic groups seems very improbable. Aside from the fact that no systemic 
mutations have ever been demonstrated, among the arguments against this ex- 
planation two seem particularly telling. It is extremely unlikely that a single 
chance mutation would cause all of the many changes in the physiology and 



296 • THE MECHANISM OF EVOLUTION 

morphology of the organism that would be necessary to produce a type suffi- 
ciently well adapted to a new mode of existence to be considered a new order. 
The differences between orders are numerous and varied and have clearly in- 
volved the reorganization of the entire genotype rather than a single mutation, 
no matter how drastic its effects. Furthermore, if systemic mutations are so 
precious and so rare, and give rise to new orders at one bound, then in sexually 
reproducing species this lone individual of the new order becomes a voice in the 
wilderness seeking its mate, which does not exist, and hence the order that 
originated at one step becomes extinct in one step. If they are frequent enough 
to occur contemporaneously, they should have been observed by now. On the 
other hand, if the mutant mates with members of the parent species, it has not 
even achieved reproductive isolation and can hardly be regarded as anything but 
a rather drastic mutation, certainly not a new order. 

The Origin of Higher Taxonomic Groups 

If quantum evolution cannot be explained by systemic mutation or other 
even less satisfactory theories, how can it be explained within the existing theo- 
retical framework and why are large gaps so common in the fossil record be- 
tween the orders and other higher taxonomic categories ? In order to discuss this 
question it seems advisable to discuss preadaptation, a word often subject to mis- 
interpretation. We shall use it, not in the sense that the organisms foresee the 
course of their own evolution and make the necessary adaptive shifts before they 
are actually needed, but rather in the sense that in the process of becoming 
adapted to existing conditions, the organisms are modified in such a way that 
they are also adapted, by chance, to some other set of conditions under which 
they have never existed. The first step in the evolution of an internal parasite, 
for example, would be the development of the ability to survive within the body 
of its host. Of necessity, this type of change would have to be preadaptive. The 
lungfish are adapted to survive in warm, stagnant waters with a low oxygen con- 
tent because the lungs enable them to obtain oxygen from air. However, lungs 
were preadaptive for terrestrial life. Hence, it appears that preadaptation can 
arise as an incidental by-product of adaptation. 

It is, therefore, entirely conceivable that numerous preadaptations may 
exist at any particular time. If a new evolutionary opportunity or ecological niche 
opens up to a preadapted population, it may occupy the new niche relatively 
rapidly though still by the gradual neo-Darwinian process involving mutation, 
natural selection, and possibly genetic drift and gene flow. The shift has been 
visualized in terms of a shift from one adaptive zone to another or from one 
adaptive peak to another, as shown in Fig. 31-3. 

Next let us consider the conditions under which evolutionary changes 



THE ORIGIN OF SPECIES • 297 

will occur most rapidly. These conditions exist when a species is subdivided into 
many relatively small, partially isolated populations. Each constitutes essentially 
a separate adaptive experiment, for divergence is not only possible but probable 
as each population adapts to its own immediate environment. Any particularly 
successful group can spread rapidly either by migration and gene flow into 
adjacent populations (since isolation is incomplete) or by winning out in inter- 
population competition. Striking new adaptive types appear most likely to emerge 
when a species range covers a diversified environment or when the environment 
itself is unstable, for then a variety of selection pressures is exerted. 

Therefore, the origin of a higher taxonomic group such as an order may 
occur in a single, rather small, preadapted population of a species to which a 
new ecological niche becomes available. The entire transition may occur in a 
relatively short time, geologically speaking, and involve relatively few individuals 
compared to the numbers of the old and new orders that lived before, after, and 
even during the transition. Viewed in this light, it is not at all surprising that so 



Time 




Adaptive 
zone 1 



Nonadaptive 
zone 



Adaptive 
zone 2 



Fig. 31-3. Quantum evolution: transformation in time leading to the origin of 
major higher categories such as orders. Note that speciation, phyletic evolution, and 
quantum evolution may go on simultaneously and that at all times the basic evolu- 
tionary unit is a breeding population. (After Simpson.) 



few transitional fossil types have been found. It becomes simply a matter of 
statistics and not a unique or mysterious process. It should be noted that the 
species is the evolutionary unit even when it is giving rise to higher taxonomic 
levels. Evolution at all levels and rates is due to changes in gene frequencies 
within breeding populations. Phyletic and quantum evolution are useful descrip- 
tive terms, but they do not imply a different mechanism of evolution. All three 



298 • THE MECHANISM OF EVOLUTION 

processes may be concurrent, and the changes may be rapid or slow, requiring 
millions of years, but the species remains the basic unit of evolution under all 
circumstances. 

The fossil record of the horse family or Equidae is probably as well 
known as that of any other group. Early horses were small browsing animals, 
feeding on the tender foliage of trees and shrubs. They evolved and diversified 
within the browsing adaptive zone. One group gave rise suddenly to the grazing 
horses, which fed on harsh grasses, but despite the wealth of fossil Equidae mate- 
rial, no intermediates are known. The preadaptive change in this case appears to 
be the development of the larger and higher crowned teeth required to grind up 
the necessary amount of vegetation to support the larger body that had evolved 
in some of the browsing horses. These teeth were preadaptive for grazing, and 
if the horse now supplemented its diet with grass, a new ecological niche was 
opened up. Since the prairie habitat was apparently not occupied by significant 
competitive herbivores, selection pressures would then be very strongly in favor 
of the transition, for competition would be keen for the browsing animals and 
slight at this point for grazing animals. The ultimate result — adaptation by the 
Equidae to two kinds of food — permitted an increase in the total number of 
existing horses. 

One point, however, should be made in relation to the models of 
quantum evolution. The species must remain well adapted during any and all 
transitions. If it did not, it would become extinct. For example, major changes 
in the form and function of the foot have not required that the members of a 
species hobble around during the transition period, which could well have been 
a million years or more. Thus, the nonadaptive zones or the nonadaptive valleys 
are misleading. One species' peak may be another species' valley; or, for a given 
species, the peak itself moves as the species evolves. 

Although much still remains to be learned, the broad outlines of the 
course of evolution and of the mechanism of evolution are now fairly well 
understood. More research on the effect one species has on its own evolution or 
on that of other species is needed. Darwin's theory of sexual selection, now more 
or less in limbo, was an attempt to study the effect of a species on its own evolu- 
tion (see Fig. 31-4). The cooperative as well as the competitive aspects of 
natural selection are decidedly in need of further study, for cooperative efforts 
may confer a reproductive advantage to a particular population in competition 
with other populations. Evolution then may reflect the effects of both cooperation 
and competition. 

A breeding population is an array of genes, temporarily embodied in 
individuals, but endlessly combined and recombined by the process of sexual 
reproduction. New genes may be added to the existing array by mutation or by 
gene flow, while random genetic drift may lead to chance fluctuations in the 
existing gene pool. Each individual, each new combination of genes, is a unique 



THE ORIGIN OF SPECIES • 299 



^Hs 




Fig. 31-4. Darwin's finches: speciation, following the initial in- 
vasion of the Galapagos Islands by finches from South America, 
has given rise to fourteen closely related but divergent species. 
(With permission of Lack.) 

adaptive experiment to be tested by natural selection. Similarly, each breeding 
population is a unique adaptive experiment to be tested by natural selection in 
competition with other populations. Although our discussion, by focusing pri- 
marily on events at a single gene locus, has oversimplified a very complex 
mechanism, it has indicated the general nature of the process of evolution. 



300 • THE MECHANISM OF EVOLUTION 

SUMMARY < 



Although many species definitions have been proposed, 
most of them can be categorized as either "morphological" or 
"biological." The morphological species definitions use the degree 
of morphological similarity as the criterion for distinguishing 
between species. The biological species definitions emphasize 
reproductive isolation as the essential criterion without regard to 
morphological traits. The biological definitions are more objec- 
tive, in that the judgment is based on the behavior of the organ- 
isms in nature rather than on the subjective opinion of a taxono- 
mist. The fundamental question is not whether the members of 
the two populations can interbreed but whether, in fact, they do. 
If they do not, they are pursuing independent evolutionary paths 
and must therefore be regarded as separate species. The nature of 
this definition makes it applicable primarily to sympatric sexually 
reproducing organisms. Three types of evolutionary change have 
been recognized — speciation, phyletic evolution, and quantum 
evolution — but all the fundamental questions about evolution are 
related to the process of speciation. Even the origin of higher 
taxonomic groups appears to have been the result of relatively 
gradual changes in the hereditary traits of an interbreeding group 
of organisms. 

SUGGESTED READING 

Amadon, D., 1950. "The Hawaiian honey creepers," Bull. Amer. Museum of 

Natural History, Vol. 95. 
Clausen, J., 1951. Stages in the evolution of plant species. Ithaca, N. Y. : Cornell 

University Press. 
Darwin, C, 1872. The origin of species. New York: Mentor Books (1958). 
Dobzhansky, Th., 1951. Genetics and the origin of species, 3d ed. New York: 

Columbia University Press. 
Lack, D., 1947. Darwin's finches. New York: Cambridge University Press. 
Mayr, E., ed., 1957. The species problem. AAAS Symp. 50. 



CHAPTER 



32 



Evolution of Genetic Systems 



Thus far we have discussed evolution almost exclusively 
in terms of sexually reproducing, diploid species. This type of 
genetic system is undoubtedly the most familiar reproductive 
mechanism because it is predominant among the higher animals 
and plants. However, it is by no means the only scheme possible, 
and many other systems are known. In view of these possibilities, 
the question may well be posed as to why sexuality and diploidy 
should have come to assume their predominant position. If evolu- 
tion and natural selection have affected the hereditary character- 
istics of organisms in such ways that they become phenotypically 
better adapted to survive, and reproduce in their physical and 
biological environments, there is no reason to suppose that the 
hereditary mechanism itself is not similarly subject to modification 
and improvement under the influence of evolutionary forces. The 
fossil record gives some clues to the course of evolution in 
morphological traits, but no similar clues are available for the 
evolution of genetic systems, and conclusions in this area are 
based primarily on inferences derived from our knowledge of 
living species. Although our surmises as to their mode or sequence 
of origin must be regarded as rather speculative, the fact that a 
great diversity of different genetic systems exists cannot be 
disputed. 

Genetic Recombination 

Except for several viruses in which RNA is utilized, the 
control and transmission of hereditary traits, from viruses up to 



301 



302 • THE MECHANISM OF EVOLUTION 

man, reside in a single type of compound, DNA. In viruses, bacteria, 
and the blue-green algae, the DNA does not appear to be organized into 
well-defined structures, comparable in organization and behavior to the chromo- 
somes of higher plants and animals. For a long time it was assumed that these 
rather simple, primitive organisms reproduced only asexually, and that sexual 
reproduction, leading to genetic recombination, had evolved from asexually re- 
producing species. However, the recent discovery of various kinds of genetic 
recombination in bacteria and viruses has reopened the question of which is the 
more primitive condition, sexuality or asexuality. 

The processes observed in these simple organisms are in several respects 
different from sexual reproduction in higher plants and animals. It should be 
noted and emphasized that sexual reproduction has very little to do with sex in 
the Freudian sense. Though separate sexes, male and female, are sometimes in- 
volved, the essence of sexual reproduction is genetic recombination. Corn and 
earthworms, for example, do not have individuals of different sex, yet they re- 
produce sexually. If those processes resulting in genetic recombination are termed 
sexual, then the unusual forms of recombination in viruses and bacteria fall 
within the realm of sexuality. 

Transformation, the artificial recombination in Pneumococcus induced 
when DNA from one strain is added to a culture of a different strain, has al- 
ready been mentioned in an earlier chapter. In Escherichia coli, the colon bacillus, 
strains have been found that regularly undergo genetic recombination during 
cellular contact. In this case, however, only part of a single "chromosome" or 
linkage group from one type of strain (F + or Hfr) enters an F~ cell to form a 
partial heterozygote. The size of the transferred fragment is related to the time 
allowed for cellular contact. Still another type of genetic recombination in bac- 
teria, known as transduction, is mediated by bacterial viruses or bacteriophages. 
In transduction, DNA from one strain of bacteria is transferred to a different 
strain by means of the phage. Thus three rather different recombination mechan- 
isms are known in bacteria: transformation, transduction, and cellular fusion. 
They differ in amount of DNA transferred (least in transformation, greatest 
with fusion) and they also differ from recombination in higher organisms in 
that less than a complete genetic complement may be involved. 

A whole new field of genetics has been opened up by the discovery that 
genetic recombination occurs in bacteriophages. Since a phage particle consists of 
a DNA core covered by a protein sheath, it is of great interest that even at this 
simple level of organization genetic recombination is possible. Since the phages 
multiply only in association with a bacterial host, recombination occurs only 
when a single bacterium harbors more than one type of virus particle. As yet 
sexual processes have not been reported in blue-green algae or in many types of 
bacteria. However, it would not be surprising if future studies reveal recombi- 
nation mechanisms in additional groups of microorganisms. 



EVOLUTION OF GENETIC SYSTEMS • 303 

The evolution of the somewhat more complex unicellular algae and 
protozoans was accompanied by a more complex and precise organization of the 
genetic material itself. The genes were organized into chromosomes within a 
nucleus, and mitosis provided for the exact distribution of a complete set of 
hereditary material to each daughter cell following asexual cell division. Simi- 
larly, meiosis insured the exact segregation and union of complete chromosome 
sets during sexual reproduction. These three advances, the origin of chromo- 
somes, mitosis, and meiosis, represent major steps in the evolution of the genetic 
material. 



Asexual versus Sexual Reproduction 

In spite of the fact that genetic recombination is known from even the 
simplest and most primitive of organisms, it is nevertheless true that asexual 
reproduction is very common among organisms at many levels of organization 
and complexity. This observation raises questions about the adaptive advantages 
and disadvantages of both asexual and sexual modes of reproduction. On the 
assumption that asexuality is the more primitive condition, then sexual repro- 
duction has arisen independently a number of times. On the contrary assumption, 
that sexuality is more primitive, then asexual reproduction has evolved repeat- 
edly. In either case, the indications are that the genetic system has adaptive value 
and has been modified during the course of evolution. Arguments and theories 
favoring both assumptions have been advanced in recent years, with perhaps a 
preponderance favoring sexuality as the more primitive state in view of the 
recent discovery of genetic recombination in viruses and bacteria. It is appro- 
priate, therefore, to consider now the adaptive significance of asexual reproduc- 
tion. 

Any asexual method of reproduction provides a means whereby rapid 
self-duplication of a particular genotype is possible. If this genotype is well 
adapted to a given stable environment, asexual reproduction is then a more effi- 
cient means of rapidly colonizing this environment and maintaining a well- 
adapted population there than is sexual reproduction. With genetic recombina- 
tion a variety of new genotypes is produced, many of which may be poorly 
adapted to the existing stable environment. Asexual reproduction will also be 
advantageous where the numbers of individuals are so small that the probability 
of encountering suitable mating partners is low. However, an asexually repro- 
ducing population is poorly equipped to adapt to rapidly changing environ- 
mental conditions. Its sole means of adapting to changed conditions is through 
the chance occurrence of rare favorable mutations. In species such as bacteria 
with large numbers and high rates of multiplication, this method of adaptation 
may be sufficient as a buffer against extinction, but in other species it is not. 

Sexual reproduction, on the other hand, through the shuffling and sort- 



304 • THE MECHANISM OF EVOLUTION 

ing of genes into new and different combinations with each generation, provides 
a constant source of new phenotypes for testing against the environment. Al- 
though at any one time and place there will be a smaller proportion of well- 
adapted individuals than would be produced by a well-adapted asexual popula- 
tion, a sexually reproducing population is better able to adjust to changing 
environmental conditions and to exploit new and different ecological niches. It is 
not surprising, therefore, that among the so-called higher or more complex 
organisms, sexual reproduction seems to be the mechanism through which this 
complexity has evolved. 

If sexual recombination is truly the more primitive mode of reproduc- 
tion, then asexual reproduction is a condition derived from it. The asexual status 
of many bacteria, protozoans, and other groups of microorganisms can then be 
interpreted as an adaptive phenomenon in these organisms. Many of them exist 
in relatively stable environments in which rapid asexual multiplication is advan- 
tageous. Others, living under unstable conditions, are nonetheless capable of such 
rapid multiplication and can adapt so readily via single mutations that asexual 
reproduction would still have an adaptive advantage over any benefits from 
genetic recombination. 

Haploidy versus Diploidy 

Although there may still be some doubt as to the primitive status of 
sexual phenomena, it seems reasonably clear that haploidy is the primitive state 
from which diploidy has been evolved in a number of different unrelated groups. 
At the level of organization above the viruses, bacteria, and blue-green algae— 
namely, the flagellates and the green algae — the genetic material is organized 
into chromosomes that undergo mitosis and meiosis. The most primitive flagel- 
lates and green algae are haploid; the only diploid cell is the zygote, and this 
cell undergoes two meiotic divisions that immediately restore the haploid condi- 
tion. In the evolution of both higher plants and animals, there has been a defi- 
nite trend toward prolongation of the diploid phase. In other words, the interval 
between fertilization and meiosis has increased, with a number of mitotic divi- 
sions of the diploid nucleus intervening before meiosis. This observation raises 
at once the question of the adaptive advantages of diploidy. 

The Metazoa and some groups of Protozoa are completely diploid ex- 
cept for the gametes; that is, meiosis is deferred until just prior to gamete forma- 
tion. In plants, a similar situation exists in the diatoms, yeasts, certain green 
algae, and some of the brown algae. Among the algae, the haploid life cycle has 
frequently given rise to an alternation of haploid and diploid generations that 
are morphologically very much alike. In this case the zygote divides mitotically 
to form the plant body, but the deferred meiosis, when it occurs, produces 



EVOLUTION OF GENETIC SYSTEMS • 305 

haploid spores rather than gametes. The spores then germinate and develop into 
a haploid organism similar in form to the diploid. It appears that from this type 
of life cycle, known as an isomorphic one, two different types of heteromorphic 
life cycles have been derived. The predominant diploid type is found in the 
vascular plants and some of the more complex brown algae. A predominantly 
haploid life cycle is found in a few algal groups and in the mosses and liver- 
worts. The early theory that the evolution of a predominant diploid generation 
made possible the invasion of the land by plants now appears to be incorrect. 
For one thing, the complex marine brown algae also have a predominant diploid 
phase, whereas many terrestrial fungi have retained the haploid condition. 
Furthermore, the bryophytes, supposedly representative of a stage intermediate 
between the haploid algae and the predominantly diploid vascular plants, are 
apparently more recent in origin than the oldest vascular plants and represent an 
evolutionary dead end rather than a transitional form. Therefore, it appears that 
the adaptive advantages of diploidy must be sought elsewhere than in its rela- 
tionship to the invasion of the land. 

In a haploid organism, the genotype, whatever it may be, is immediately 
expressed. All of the genotypes in a population are exposed to selection at all 
times, and little variability can be retained since all mutants unfavorable at the 
moment will be eliminated. A diploid, however, may carry a considerable amount 
of unexpressed variability in the form of recessive genes in the heterozygous 
condition. A portion of this variability will be released and exposed to selection 
each generation owing to genetic recombination. In this way a population retains 
its ability to adapt to changing environmental conditions while at the same time 
remaining well adapted to the prevailing conditions. The flexibility should not 
be regarded as simply dependent upon the appearance of new homozygous re- 
cessive mutant types, however, for diploidy also opens up the possibility for 
interallelic, epistatic, and heterotic effects which may be of considerable im- 
portance. In general diploidy is associated with the more complex organisms 
that have a long, precisely integrated sequence of development. In haploids, evo- 
lution is primarily dependent upon the appearance of suitable favorable muta- 
tions. Diploidy, through gene recombination and interaction, permits the forma- 
tion of new and different integrated systems of genes without serious loss of 
fitness. The effects of most single gene mutations on a complex developmental 
sequence are deleterious, and in organisms with low reproductive rates and a 
long developmental period, favorable individual mutations would customarily be 
too rare to give adequate adaptive flexibility. Thus, diploidy would appear to be 
an adaptive means of conserving and releasing variability in higher organisms. 
In the mosses and liverworts the predominant haploid gametophyte may have 
evolved in relation to their pioneering tendency, for a well-adapted initial in- 
vader can quickly produce a colony of similarly well-adapted descendants. 



306 • THE MECHANISM OF EVOLUTION 

The Separation of the Sexes 

We have already seen that genetic recombination has been observed in 
even the simplest of organisms. The evolution of organisms of greater com- 
plexity has been accompanied by the evolution of more complex systems for 
ensuring sexual reproduction. In the Protozoa, two types of sexual process are 
known. In conjugation, a temporary contact between two protozoans — for ex- 
ample, paramecia — permits nuclear exchange. In syngamy, an actual fusion of 
sex cells or gametes takes place to form a zygote. In some cases the fusing 
gametes, known as isogametes, are identical in size and form to, and little dif- 
ferent from, the parent cells. In other species, the sex cells, called anisogametes, 
are similar in form but quite different in size, while in still others differentiation 
of the gametes into sperm and egg cells has occurred. All of these types of repro- 
duction have been observed in one flagellate group, the Phytomonadina, and 
suggest how the differentiation of sex cells could have taken place. 

In the colonial flagellate, Volvox, a. single colony is capable of pro- 
ducing both sperm and egg cells. The production of two kinds of gametes, sperm 
and egg, by a single individual is known as hermaphroditism. Hermaphrodites 
are found throughout the plant kingdom, though some plants such as willows or 
the ginkgo have separate sexes. Hermaphroditism is widespread among animals 
though not so common as in plants; in such important groups as nematodes, 
insects, and vertebrates it is rare or absent. Because it is so common, particularly 
among the lower animals and plants, it appears that hermaphroditism, among 
multicellular animals and plants at least, is the ancestral condition from which 
the separation of the sexes has been derived. Furthermore, the separation of the 
sexes has even been obtained experimentally in hermaphroditic species — for 
example, in corn — through the suppression of functional male flowers in one 
type of plant and functional female flowers in another. (Species with separate 
sexes are frequently referred to in the literature as bisexual, an unfortunate and 
confusing choice of terms since bisexual is synonymous with hermaphroditic.) 

Sex Determination 

In hermaphrodites, such as corn or an earthworm, male and female sex 
cells are produced by an individual with a single genotype. In this case sexual 
differentiation cannot be determined genetically, but rather by subtle differences 
in the internal environment comparable to those leading to the differentiation of 
other organs of the body. 

In species with separate sexes, a variety of methods of sex determination 
have evolved. Here, too, environmental sex determination occurs. The best- 
known example comes from the marine echiurid worm, Bonellia. If the free- 
swimming larva, when it settles to the sea bottom to undergo further develop- 



EVOLUTION OF GENETIC SYSTEMS • 307 

ment, happens to land on the proboscis of a female, it will enter the body of the 
female where it differentiates into a minute male, living a parasitic existence in 
the nephridium near the uterus. If the larva lands on the sea bottom, it differ- 
entiates into a free-living female some 500 times as large as the male. The en- 
vironmental nature of sex determination in this species can be demonstrated by 



lobes of 
proboscis 




ciliated groove 



male on 
proboscis 



mouth 



body 



anus 



Fig. 32-1. The echiurid marine worm Bonellia, 

showing the vast size difference between the 

sexes despite environmental sex determination. 

(With permission of Begg.) 



rearing larvae in sea water containing female proboscis extract. All of the larvae 
then become males. See Fig. 32-1. 

In the majority of species with separate sexes, sex determination has 
been brought under genetic control. A number of different types of genetic sex 
determination have been identified. The most familiar type involves a hetero- 
gametic male. In this situation the male carries two different kinds of sex chro- 
mosomes, the X and Y, and produces two kinds of sperm, bearing either an X 



308 • THE MECHANISM OF EVOLUTION 

plus the autosomes, or a Y plus the autosomes. The XX females produce only 
one type of egg, having a single X and a set of autosomes. A variation is found 
in some species in which the females are XX and the males XO, having one less 
chromosome than the females. 

In the heterogametic female type of sex determination, it is the female 
that has two different kinds of sex chromosomes, conventionally called Z and W. 
Consequently, the female produces two kinds of eggs. Here, too, a ZO modifica- 
tion has been demonstrated in some species. Heterogametic females are found in 
moths and butterflies, in birds, and in some fishes. Heterogametic males are 
found in most other groups with separate sexes. 

The work of Bridges on sex determination in Drosophila melanogaster 
led to the development of the balance theory of sex determination. As a result 
of his findings he concluded that the presence of two X chromosomes was not 
alone sufficient to determine femaleness nor were an X and a Y sufficient for 
maleness. Rather, sex was influenced by the autosomes as well as the sex chromo- 
somes and the significant feature was the ratio of X chromosomes to haploid sets 
of autosomes. The basis for his conclusion was a study of the sexual character- 
istics of flies with abnormal numbers of sex chromosomes and autosomes. Some 
of the types he obtained were as follows : 



chromosome complement 






(X = X chromosomes; 


ratio 




A = sets of autosomes) 


X/A 


Phenotype 


3X : 2A 


1.5 


superfemale 


3X : 3A 


1.0 


normal triploid female 


2X : 2A 


1.0 


normal diploid female 


2X : 3A 


0.67 


intersex 


IX : 2A 


0.50 


normal male 


IX : 3A 


0.33 


supermale 



Of particular interest is the intersex shown above. It has two X chromosomes, 
but is not a normal female since the balance between sex chromosomes and auto- 
somes has been upset. All of the types observed were consistent with the rule 
that an X/A ratio of 1.0 or above resulted in a female (normal or super) and a 
ratio of 0.5 or below in a male (normal or super). Ratios between 0.5 and 1.0 
produced intersexes, showing varying admixtures of male and female traits. The 
subsidiary role of the Y chromosome in Drosophila is shown by the fact that an 
XXY individual with two sets of autosomes is a fertile female: 

In the bryophytes (the mosses and liverworts) a somewhat different 
type of chromosomal sex determination has been observed — the heterozygous 
sporophyte. In this case the diploid sporophyte is neither male nor female but 
carries an X and a Y chromosome as well as the autosomes. The spores produced 
by the sporophyte are of two kinds: X-bearing spores develop into female 
gametophytes; Y bearing, into male gametophytes. 



EVOLUTION OF GENETIC SYSTEMS • 309 

Even more significant is the type of sex determination exemplified by 
Melandrium album, a member of the pink family. In this species, some plants 
bear only male flowers and others only female flowers. The females have two X 
chromosomes plus two sets of autosomes; the males, an X and a Y chromosome 
in addition to the autosomes. However, sex determination in polyploids of 
Melandrium has shown the mechanism to be different from that in Drosophila. 
In Melandrium, the Y chromosome is male determining. As long as the Y is 
absent, any ratio of X to A in diploids, triploids, or tetraploids will produce 
fertile female plants and no intersexes. A single Y is sufficient to produce male 
plants even in triploids and tetraploids. Thus, for example, the following types 
are all male plants : 

diploid 2A— X— Y 

triploid 3A— X— 2Y 

3A— 2X— Y 

tetraploid 4A— 2X— 2Y 

4A— 3X— Y 

None are intersexes, though occasionally a male plant will bear an hermaphro- 
ditic flower. Thus, quite a different use is made of the XY mechanism in 
Melandrium and in Drosophila. In Melandrium, the X chromosome seems to 
bear genes for femaleness, the Y carries genes for maleness, and the autosomes 
are without apparent influence on sexuality. In Drosophila, the factors for 
femaleness seem to be borne on the X chromosomes, those for maleness on the 
autosomes, and the Y, aside from an effect on fertility, seems to have little influ- 
ence. The work on Melandrium has recently assumed new interest with the dis- 
covery that sex determination in mice and men, and probably in other mammals, 
is similar to that in Melandrium and not like that in Drosophila. This conclusion 
is based on the discovery that sterile human females with a condition known as 
Turner's syndrome are XO diploids. A fruit fly of this constitution would be 
phenotypically male. Furthermore, sterile human males with Klinefelter' s syn- 
drome are XXY and diploid for the autosomes. As mentioned above, in 
Drosophila such individuals are phenotypic females and not males. Thus in man 
the Y chromosome is male determining. 

In the Hymenoptera (the ants, wasps, and bees), still another type of 
sex determination exists. Here the female is diploid; the male, haploid. The sex 
of an individual depends upon whether the egg is fertilized. Fertilized eggs 
develop into females; unfertilized eggs develop parthenogenetically into haploid 
males. Hence, whereas in most groups the sex ratio is fixed, in the Hymenoptera 
it may vary considerably. In the social insects especially, a great preponderance of 
females may be produced. A haploid male receives a single haploid set of chro- 
mosomes from his mother and passes it intact to all of his daughters; he has no 



310 • THE MECHANISM OF EVOLUTION 

father, and he fathers no sons of his own. The first meiotic division is abortive; 
the second produces two identical functional sperm. In fact, all of his sperm cells 
are genetically the same, for there can be, of course, no synapsis or crossing over. 

Sexual Differentiation 

In the honey bee there are two kinds of females, the workers and the 
queens. The workers ordinarily do not reproduce, but the queen mates and lays 
the eggs for the entire colony. Genetically, the queens and workers are the same. 
The differences in morphology and fertility between them have been traced to 
the kind of food they receive as larvae. Larvae destined to become queens are 
fed royal jelly, a food far richer in pantothenic acid, a vitamin, than the food 
given to worker larvae. The honey bee provides an insight into the relationship 
between sex determination and sexual differentiation. Even though both workers 
and queens are genetically determined females, the workers are sterile and only 
the queens become functional females. The sexual differentiation of the two 
groups is modified by environmental factors. Hence, although in species with 
chromosomal sex determination the sex of the individual is determined at the 
time of fertilization, subsequent events may modify or even inhibit normal 
sexual differentiation. 

A variety of influences may affect sexual differentiation to the extent 
that sexual anomalies result. The Drosophila intersexes resulting from chromo- 
somal imbalance have already been mentioned. They show a curious blending of 
male and female traits, the gonads and the secondary sexual characteristics being 
intermediate in form. Another quite different type of intersex is the gynandro- 
morph. In these peculiar individuals, one part of the body is male and the other 
is female. The most striking cases have been found in insects because the insects 
evidently do not have an endocrine system responsible for the circulation of sex 
hormones throughout the body. A clear-cut line of demarcation exists between 
male and female sectors. Thus, each cell is autonomous with respect to its sexual 
differentiation. The differences arise when developmental accidents lead to differ- 
ences in the sex chromosome complement in different body regions. In Dro- 
sophila, occasional individuals are male on one side and female on the other 
(Fig. 32-2). These individuals began as genetic females, but the loss of an X 
chromosome from one of the nuclei at the two-cell stage resulted in the gynan- 
dromorph. 

One of the more surprising phenomena in sexual differentiation is sex 
reversal. Frogs and toads are particularly subject to this type of transformation. 
For example, it was found that a temperature of 32° C during development 
would cause genetically female frogs to develop into fertile males. It then be- 
came possible to mate two genetic females, one a normal XX female, the other 
also XX but male. Since only X-bearing gametes are possible, only female off- 



EVOLUTION OF GENETIC SYSTEMS • 311 




Fig. 32-2. A gynandromorph in Drosophila, the left half 
female, the right half male. On the male side, note the sex 
comb on the right foreleg, the dark tip to the abdomen and 
the mutant trait, singed bristles, all of which are absent from 
the female half. (With permission of Stern. ) 

spring should result under normal developmental conditions; and indeed, among 
a large progeny, no males were found. 

A rare situation in chickens offers an even more spectacular type of sex 
reversal. In these cases a normal hen gradually assumed the appearance and be- 
havior of a rooster and actually fathered chicks. Only one ovary in a normal hen 
is functional, and when this ovary was destroyed, the primary sex cords in the 
other vestigial gonad differentiated into a testis. The male sex hormone from the 
testis then induced the changes in the secondary sexual traits (Fig. 32-3). 

Another example of the role of the sex hormones in sexual differentia- 
tion in vertebrates comes from cattle. When twin calves of opposite sex are born, 
the female is almost always sterile and is called a "freemartin." The sex organs 
are usually modified, and in extreme cases the ovaries have been transformed 
into structures resembling testes. In twin cattle, fusion (anastomosis) of the 
placental blood vessels occurs and so to some extent their bloods are mixed. 
Since the hormone system causing male differentiation comes into play somewhat 
earlier than the female system, the female twin is affected by the male's hormones 
before her own hormonal system becomes effective. The female is transformed 
into an hormonal intersex but does not become a functional male. 



312 • THE MECHANISM OF EVOLUTION 




Fig. 32-3. Sex reversal. A female fowl whose ovary was removed when 
thirteen days old resembles at maturity a typical cock. (Courtesy of Snyder 

and David.) 

Still another type of intersex has been discovered in the gypsy moth, 
Lymantria dispar. Crosses between males and females from the same locality 
produce normal male and female offspring. However, crosses between individuals 
from different races sometimes result in intersexes as well as normal progeny. 
In these moths the female is heterogametic and the male-determining factors 
seem to reside on the Z chromosomes. The female-determining factors appear 
to be carried by the W chromosome, the autosomes, and perhaps in the cytoplasm. 
In different races the effectiveness of these factors in determining sex varies, so 
that some races are "weak" and others "strong." For example, if a "weak" Euro- 
pean female is crossed to a "strong" Japanese male, the sons are normal but the 
daughters intersexual. The single "strong" Japanese Z chromosome is sufficient 
to overcome the effects of the female-determining factors so that the ZW indi- 
viduals differentiate into intersexes rather than females. The F 2 from this cross 
again produces normal sons, but the daughters are half normal and half inter- 
sexual. The reciprocal cross, "strong" Japanese female with "weak" European 
male, gives a normal F x , but in the F 2 the daughters are normal, while half the 
sons are intersexes and half normal. Here again as in Drosophila a balance be- 
tween factors of opposite effect is essential to normal sexual differentiation. 
However, in Drosophila the intersexes resulted from chromosomal imbalance. In 
Lymantria all of the individuals are diploid, and the intersexes result from a 
genie imbalance. Therefore, it must be concluded that the factors regulating 



EVOLUTION OF GENETIC SYSTEMS • 313 

normal sexual differentiation have been mutually adjusted in the different races 
of the gypsy moth by many generations of natural selection. 

This brief review of sex determination and sexual differentiation is in- 
tended to show that an individual is not irrevocably one sex or the other. Every 
cell appears to have the potential to become either male or female in its charac- 
teristics. The sex that actually develops depends upon the type of reaction system 
that is set up in the cell. If one system is brought into play, a male develops; the 
other produces a female. The factor determining which system will prevail may 
be environmental, as in Bonellia, or it may be genetic, as in the familiar chro- 
mosome mechanism of sex determination. If the sex-determining machinery itself 
is thrown out of kilter — for example, because of chromosomal imbalance — ab- 
normal sexual development will ensue. However, even if the sex-determining 
mechanism operates normally, this may not be sufficient to insure normal sexual 
differentiation, for unusual environmental influences such as hormones, tempera- 
ture, nutrition, etc., may modify differentiation to the extent that intersexes or 
sexually aberrant individuals result. 

The Control of Recombination 

From an evolutionary standpoint the separation of the sexes into male 
and female individuals may be regarded as a means of insuring cross fertilization 
and genetic recombination. A comparative examination of the genetic systems in 
numerous groups of plants and animals reveals a wide range in the amount of 
recombination. The available evidence suggests that recombination itself is under 
the control of natural selection, and that the differences between groups in the 
amount of recombination are adaptive. 

Numerous mechanisms are known to increase recombination. Meiosis 
provides for a regular segregation and reassortment of the chromosomes, and a 
high chromosome number and a high frequency of chiasma formation will also 
increase the amount of recombination taking place. The separation of the sexes, 
of course, makes cross fertilization mandatory, but even in hermaphrodites, 
devices that reduce or prevent selfing are common. Differences in time of matura- 
tion of the gametes, or flower structures that make self-pollination unlikely are 
cases in point. Species with reciprocal cross fertilization often have the male and 
female reproductive tracts completely separated. Systems of self-sterility alleles 
also prevent self-fertilization in many species. More or less permanent hybridity, 
which appears in many cases to take advantage of heterotic effects, is maintained 
by systems of balanced lethals, inversion or translocation heterozygotes, or by 
allopolyploidy. 

On the other hand, several factors are known that tend to reduce or 
suppress recombination. The organization of the genetic material into linkage 
groups in the chromosomes prevents free recombination among genes. The 
smaller the number of chromosomes, the greater the restriction on recombination. 



314 • THE MECHANISM OF EVOLUTION 

Furthermore, reduction in chiasmata frequency will still further limit genie re- 
combination. Interference in regions adjacent to a chiasma limits the number of 
crossovers and hence the amount of recombination possible within a linkage 
group in any one generation. Thus, integrated gene complexes will not be com- 
pletely disrupted by crossing over. In Drosophila, not only are the chromosome 
numbers low, but crossing over is completely suppressed in the males so that 
recombination between homologous chromosomes is possible only in the females. 
Structural hybridity for inversions or translocations may effectively prevent re- 
combination within the affected chromosome pairs. However, the cross-over 
frequency is often increased in other chromosome pairs in the presence of a 
structurally heterozygous pair. In this way recombination within the chromosome 
complement can be brought under quite specific control by natural selection. 

Self-fertilization will also, of course, reduce the frequency with which 
new gene combinations are formed. The effect of selfing is to increase the fre- 
quency of homozygotes in the species population. The recessive mutations as 
well as the dominants are soon brought to expression and exposed to natural 
selection. The elimination of the less well-adapted types results in a loss of vari- 
ability, which is replenished only by mutation and not by recombination. A self- 
fertilizing species then sacrifices evolutionary plasticity in favor of immediate 
fitness, and forms a complex of relatively homozygous individuals no longer 
capable of gene exchange. In hermaphroditic species, a range of conditions may 
be found from virtually complete self-fertilization to obligatory outcrossing. 
The cross sterility observed in numerous instances is one way in which inbreed- 
ing is enforced. The range of possibilities for breeding systems in hermaphro- 
dites suggests that their modes of reproduction have been adaptively modified. 
In general, it appears that the various devices leading to selfing are of more 
recent origin and represent a method for restricting recombination. 

The suppression of recombination is even more effective in species re- 
producing asexually. Asexual methods of reproduction have arisen independently 
in various ways and in many different groups of sexually reproducing plants and 
animals. Apomixis is the term used to describe a variety of kinds of asexual 
process in which the outward appearance of sexual reproduction is retained but 
no fertilization occurs. Parthenogenesis refers specifically to the development of 
unfertilized eggs. Asexual reproduction in animals frequently occurs by means of 
parthenogenesis, though budding or fission is characteristic of certain groups. 
(Though often classified as sexual, parthenogenesis in effect more nearly resem- 
bles asexual reproduction.) In plants, many additional types of asexual repro- 
duction are known: adventitious buds, bulblets, and stolons, in addition to the 
apomictic formation of seeds not only by parthenogenesis but also from various 
types of somatic cells. Many species combine the advantages of sexual and 
asexual reproduction. In the aphids, for instance, cyclical parthenogenesis per- 
mits a very rapid build-up in numbers during the favorable warm summer 



EVOLUTION OF GENETIC SYSTEMS • 315 

months. Since every individual is a female and reproduction is not delayed until 
after mating, the reproductive potential of such a population is almost inevitably 
greater than that of a population containing both males and females. In the fall, 
a sexual generation intervenes, and from the fertilized eggs emerge the females 
that start the parthenogenetic phase once again the following spring. 

The various types of asexual reproduction are similar to self-fertilization, 
in that groups of individuals of identical genotype are formed that no longer are 
capable of gene exchange with members of other groups. They are dependent 
upon mutation for further evolution. However, unlike species where selfing is 
the rule and homozygosity is the norm, asexual methods of reproduction ordi- 
narily preserve the heterozygosity intact from one generation to the next. The 
descendants of a single individual will all have the same genotype and form a 
clone, but this particular genotype may be highly heterozygous. In fact, one 
advantage of asexual reproduction is its preservation of heterotic or otherwise 
favorable gene combinations, or of favorable chromosome combinations, aneu- 
ploid or polyploid, which are meiotically unstable. 

Generally, the changes in the genetic systems that result in the restriction 
or elimination of recombination have taken place in species where immediate 
fitness and a high reproductive rate are at a premium. There are three major 
mechanisms that limit recombination: a reduction in chromosome number and 
chiasma formation, a shift toward self-fertilization, and the development of 
asexual methods of reproduction. These devices, which lead to similar results, 
are apt to be mutually exclusive. If one type of mechanism prevails within a 
group — for example, self-fertilization — it is unlikely that the others will be 
found to any significant degree within the same group. Furthermore, the retreat 
from the cross fertilizing, diploid condition, though it confers immediate adap- 
tive advantage and fitness, does so at the expense of long-range adaptability. The 
loss of the flexibility made possible by genetic recombination seems destined to 
lead ultimately to the extinction of those groups that travel too far down this 
path, for they will be unable to cope with or adapt to changing environmental 
conditions. 

Sexual Selection 

In 1871 Darwin published a work entitled The descent of man and 
selection in relation to sex. In this book he set forth his opinions on the origin 
and evolution of man, a subject he had deliberately dismissed with just a sen- 
tence in The origin of species, in the hope that he would thereby not add to the 
prejudices against his views. Darwin's writings on human evolution are still 
cited rather regularly. However, the greater part of this book was actually de- 
voted to sexual selection, and his theories in this area have generally been either 
rejected or ignored. It seems clear that he regarded the theory of sexual selection 



316 • THE MECHANISM OF EVOLUTION 

as almost equal in importance to the theory of natural selection. As he put it, 
"Sexual selection depends on the success of certain individuals over others of the 
same sex, in relation to the propagation of the species; whilst natural selection 
depends on the success of both sexes, at all ages, in relation to the general con- 
ditions of life." One reason the theory of sexual selection has received so little 
attention is that it is now realized that sexual selection is merely one aspect of 
natural selection. Today natural selection is denned in terms of reproductive fit- 
ness. Those genes conferring fitness, whether they contribute to survival or to 
mating success, in the final analysis tend to increase in frequency in subsequent 
generations in much the same way. Thus, sexual selection is comparable in its 
effects to differential viability, longevity, or fecundity, and can quite properly 
be grouped with them as one of the elements in natural selection. 

A second reason for the rejection of sexual selection is that Darwin 
postulated that it came about in two ways, through male competition or through 
female choice. These two intrasexual selective mechanisms have been subject to 
strong criticism ever since they were first proposed: female choice, primarily be- 
cause it is anthropomorphic; male competition, because in many species there is 
little evidence that the male successful in competition with other males neces- 
sarily leaves more progeny. 

Nevertheless, the phenomena that led Darwin to formulate the theory 
of sexual selection still remain, but little progress has been made toward a more 
adequate theory or a better understanding of the facts. The trend in the evolution 
of the higher animals has been toward sexually reproducing species with the 
sexes separate. In most such species, sexual dimorphism prevails, which in some 
cases is quite striking. Darwin's proposal was an attempt to account for the 
origin of sexual dimorphism. As such, it is undoubtedly inadequate. However, 
the significant aspect of his theory is its emphasis on the fact that the appearance 
and behavior of individuals can influence the course of evolution through their 
effects, via the nervous system, upon other organisms. Thus, the behavior and 
appearance of an individual not only affects its own chances of survival, but also 
influences the activity, behavior, survival, and reproduction of other individuals. 
The evolution of the nervous system thereby added a new dimension to evolu- 
tion. Darwin's theory was inadequate, not so much because it was wrong, but 
because it was incomplete. In polygamous species especially, male competition 
may have played a significant role in the evolution of males larger and better 
equipped for combat than the females (for example, in deer and seals). To some 
extent, female "choice" may also be significant, in the sense at least that the 
male with the more effective courtship pattern will have greater success in gain- 
ing the acceptance of the female as a sexual partner. However, these possibilities 
are but two among many that could lead to sexual dimorphism. The allesthetic 
traits, as they have been called, which become effective via the nervous systems 
of other organisms, serve a variety of functions in addition to sexual selection. 



EVOLUTION OF GENETIC SYSTEMS • 317 

Even with respect to reproduction these traits have functions other than influenc- 
ing female choice or success in male competition. For example, various stimuli 
serve to bring the sexes together. Male moths are attracted to the females over 
considerable distances by their scent, which is species specific. The calls of male 
frogs and toads in their breeding ponds and of male birds on their nesting ter- 
ritories are comparable in advertising their presence and attracting the females. 
Furthermore, the elaborate courtship patterns involving a complex sequence of 
stimuli and responses between male and female serve for attraction, sexual recog- 
nition, synchronization of mating behavior, and arousal to the peak necessary for 
the successful completion of coition. Even ovulation has been shown in many 
species to be dependent upon not just hormonal stimuli but on the interplay 
between hormonal stimuli and the nervous stimuli set off by courtship and 
mating. Those traits in males and females that are epigamic — that is, contribute 
to the successful union of the gametes — will have adaptive value and will tend 
to be favored by selection. 

One of the fundamental problems in the origin of secondary sexual 
dimorphism is genetic and developmental. The differences between males and 
females are known to be due in mammals to the influence of the endocrine sys- 
tem during development. In insects, cellular autonomy exists with respect to 
sexual differentiation. Furthermore, it is known that the genotypes of males and 
females are, to a very large extent, the same, for the autosomes are identical in 
both sexes. The genetic differences may be merely haploidy versus diploidy, one 
X versus two X chromosomes, presence or absence of a Y; or, some seemingly 
trivial environmental difference may determine which path sexual development 
will follow. The problem, very simply, is to explain the origin of the very con- 
siderable differences between the sexes when the genetic differences between 
males and females are so slight. Sexual differentiation is rather well understood, 
for example, at the level of hormonal control. The initiation and regulation of 
sexual development under the control of pituitary and gonadal hormones has 
been extensively studied experimentally. However, at the level of gene action, no 
comparable knowledge is available. The nature of the genetic control that brings 
one developmental system into play rather than the other is not at all well under- 
stood and poses a particularly difficult problem since to a large extent the same 
genetic material is responsible in each case. This area of developmental genetics 
seems to hold problems of considerable interest from the standpoint of genetics, 
embryology, and evolution. 

In addition to their epigamic functions, the allesthetic traits may pro- 
mote conspicuousness or, quite the reverse, be cryptic in function. Most epigamic 
traits, whether behavioral or morphological, are conspicuous, and these same 
traits may sometimes serve other functions. In threatening another male invading 
his territory, for example, a brightly colored male may use the same colors in the 
threat display as he uses in the courtship display before the female. Conspicuous 



318 • THE MECHANISM OF EVOLUTION 

traits have evolved not only in relation to threat but also for use as warning 
signals. The various aspects of group behavior, too complex to be detailed here, 
but including care of the young, colony and flock formation, cooperation of 
various sorts, and the social behavior of insects, are built upon intricate and care- 
fully integrated systems of interactions among individuals, and are mediated by 
the nervous system. These behavorial systems have emerged as a product of evo- 
lution. The relatively inflexible behavior patterns that we call instincts are clearly 
under hereditary control. The capacity to learn, also an evolutionary product, 
makes possible more flexible behavior patterns that can be modified as the result 
of experience. 

Cryptic behavior and form have also resulted from the operation of 
evolutionary forces. The ability to select a favorable habitat or resting place, 
cryptic behavior such as shadow elimination, and mimicry and cryptic coloration 
— all have evolved as the result of natual selection favoring those individuals 
best able to avoid perception by their enemies. In the light of these few ex- 
amples, to which so many more could be added, there can be little doubt that 
Darwin, in his theory of sexual selection, was on the track of a significant phase 
of evolution, the psychological or ethological aspect. The course of evolution in 
animals has been greatly influenced by the interactions that occur among indi- 
viduals and are mediated by the sense organs and the nervous system. A killdeer, 
when its nest is threatened by an intruder, dramatically feigns injury. Anyone 
who has ever been deceived and led astray by such a display can hardly fail to 
be impressed by the subtlety and power of the forces of evolution. 

SUMMARY < 

A major thesis of this chapter is that not only organisms 
but their underlying genetic systems have undergone evolutionary 
change and that the genetic system itself may have adaptive value. 
With few exceptions the hereditary material in living things is 
deoxyribonucleic acid (DNA). A variety of methods of genetic 
recombination have been discovered, from the novel types de- 
scribed in viruses and bacteria to the orderly system in higher 
plants and animals. This orderliness became possible with the 
organization of the genes into chromosomes that undergo regular 
mitotic and meiotic cell divisions. Asexual reproduction is espe- 
cially well suited to the rapid self-duplication of a particular 
genotype, and thus is favorable to the maintenance of a well- 
adapted genotype in a stable environment or to rapid colonization. 
A sexually reproducing population, on the other hand, is better 
able to adjust to changing environmental conditions and to ex- 
ploit new and different ecological niches. Haploidy is the more 
primitive condition, whereas the predominance of the diploid 



EVOLUTION OF GENETIC SYSTEMS • 319 

generation is associated with the evolution of organisms of con- 
siderable complexity. The evolution of sex has led to the evolu- 
tion of numerous methods for controlling sex determination and 
sexual differentiation. Sexual anomalies may result when either of 
these processes is disrupted. In sexually reproducing species, the 
amount of genetic recombination is regulated in a variety of ways, 
which range from self -sterility or enforced outcrossing to self- 
fertilization. The release of genetic variability appears to be under 
rather precise control. Darwin's theory of sexual selection, though 
inadequate in many respects, seems to merit further study, for it 
focuses attention on the fact that the appearance and behavior of 
an individual not only affects its own chances of survival but also 
influences the activity and behavior, survival and reproduction of 
other individuals as well. 

SUGGESTED READING 

Darlington, C. D., 1958. The evolution of genetic systems. New York: Basic Books. 

Stebbins, G. L., I960. "The comparative evolution of genetic systems," Evolution 
after Darwin, Vol. 1, The evolution of life. Chicago: University of Chi- 
cago Press. 




f 



ww% 







PART 



IV 



Evolution 
and Man 



CHAPTER 



33 

Human Evolution 



The Mammalia are a class of vertebrates or back-boned 
animals characterized by mammary glands, hair, and body temper- 
ature regulation. The subclass Eutheria, or placental mammals, 
bear living young that undergo a period of development within 
the uterus of the female. The Primates are placental mammals 
with elongated limbs and enlarged hands and feet, each with five 
digits. The digits have nails rather than claws or hoofs, and the 
thumb and the great toe are usually opposable to the other digits. 
Primates are generally arboreal and are found primarily in tropical 
and subtropical regions. Their orbits are directed forward so that 
they have binocular vision. Except for the highly developed brain 
and nervous system, the Primates are a relatively generalized 
group. Any objective analysis of human traits will lead inevitably 
to the conclusion that man is a vertebrate, a placental mammal, 
and a primate. He differs from other primates primarily in his 
enlarged brain and erect posture. He is cosmopolitan rather than 
tropical, terrestrial rather than arboreal, and the great toe is not 
opposable. His mastery of the arts of making fire and clothing 
first permitted him to extend his range beyond the tropics, and 
without these he would once again be a tropical species. The un- 
usual size of the great toe and shape of the foot are clear indica- 
tions of his ancestors' descent from trees in the not too remote past. 
The Primates have been classified as shown in Table 33-1. 

The Prosimians 

The most primitive, generalized mammals such as shrews 
and moles belong to the order Insectivora, from which all other 



323 



324 • EVOLUTION AND MAN 



TABLE 33-1 
The Primates 



Suborder Superfamily Family 



Common 
name 



Distribution 



Remarks 



orders of mammals are thought to have descended. For many years the tree 
shrews were included among the insectivores. More recently, however, they have 
been grouped with the primates, for even though conforming to the basic mam- 
malian plan, they show in their slightly enlarged brains and eyes the beginnings 
of primate traits. Superficially, the tree shrews resemble squirrels, for they are 
small, bushy-tailed animals that are active by day. They possess claws rather than 
nails, but their simple incisor teeth are quite different from those of the squirrels, 
which are typical of the chisellike gnawing incisors of the rodents. Their digits, 
their eyes, and their brain separate them from the insectivores and place them 
with Primates. Thus the Primates, the order to which man belongs, are linked 
directly through the tree shrews to the oldest group of mammals. Certainly in 
this instance, there is no reason to speak of a "missing link." 

The true lemurs and the aberrant aye-aye are found now only on the 
island of Madagascar, but formerly they ranged over much of the Old World 
and North America. About the size of a mouse or a cat, the lemurs are usually 
both arboreal and nocturnal. Although they display primate characteristics, they 
are rather foxlike in appearance due to their elongated, moist muzzles and 
rather large, mobile ears. Their brains, compared to those of monkeys or men, 
are relatively simple, for the cerebral cortex is small and smooth, lacking the 
folds that greatly increase the surface area in the higher primates. 



Prosimii 


Tupaioidea 




Tree shrews 


Oriental 


6 genera. Mod- 


lower 










erate number 


Primates 










of species 




Lemuroidea 




Lemurs 


Madagascar 


19 species 




Daubento- 




Aye-Aye 


Madagascar 


1 species 




nioidea 












Lorisformes 




Loris, galagos, 
bush babies, 


Africa and 
Oriental 


10 species 




Tarsi if ormes 




pottos 
Tarsiers 


East Indies 


3 species 


Anthropoidea 


Ceboidea Cebidae 


New World 


New World 


12 genera, 


higher 






monkeys 


tropics 


140 species 


Primates 














Cal 


Marmosets 




2 genera 






thricidae 






several species 




Cercopi- Cercopi- 


Old World 


Old World 


16 genera, 




thecoidea 


thecidae 


monkeys 


tropics except 
Australia 
Old World 


200 species 






'Pongidae 


Apes 


10 species 










tropics except 






Hominoidea 






Australia 








Hominidae 


Man 


Cosmopolitan 


1 species, 
Homo sapiens 



HUMAN EVOLUTION • 325 

The Lorisiformes include species with such appealing names as bush 
baby and potto, and are in general rather like the lemurs. The lorises of Asia are 
slow-moving climbers with relatively large eyes and a shorter snout than most of 
the true lemurs. The galagos or bush babies native to Africa are small and active, 
with their hind legs specially adapted for jumping. 

The tiny tarsiers, the size of small kittens, though formerly found in 
much of the Old World and North America, today live only in the East Indies. 
They have an unusual combination of primitive characters that link them to the 
lemurs, and advanced traits that suggest relationship to the monkeys. The tarsiers 
have a short face with relatively enormous eyes facing to the front, undoubtedly 
an adaptation to their nocturnal, arboreal habits. Their limbs and feet are spe- 
cially modified for both grasping and jumping, so that they flit through the 
trees with surprising ease. The tarsier's large brain, well-developed senses of 
vision and hearing, and the structure of nose and lips all suggest relationship to 
the monkeys, but his fossil relatives show him to be more closely related to the 
lemurs. 

It is of particular interest that within the rather heterogeneous sub- 
order Prosimii, the animals range in kind from the tree shrews, which are not 
far removed from the most primitive placental mammals, the insectivores, to the 
tarsiers, which foreshadow the monkeys and the other Anthropoidea (see Fig. 
33-1). 

The Higher Primates 

The higher primates, including the monkeys, apes, and man, belong to 
the suborder Anthropoidea. Though called "higher," there is not much that is 
strikingly different about them as compared to the lower primates. The differ- 
ences, however, are of considerable significance. In particular, their eyes show 
several changes that permit superb vision. The yellow spot, or macula lutea, in 
the retina directly opposite the pupil is a region of especially acute sight. 
Furthermore, the color vision of the Anthropoidea is superior to that of any of 
the other mammals. The placement of the eyes, in sockets facing directly for- 
ward, permits both eyes to cover the same field of vision. This arrangement 
differs greatly from that of a deer, for example, where each eye has a separate 
field of vision with relatively little overlap. The higher primates are thus able to 
see not only clearly and in color but also in three dimensions. The effect of 
binocular vision is similar to that of an old-fashioned stereopticon, for each eye 
views an object from a slightly different direction, and the object seems to stand 
out in three dimensions so that very accurate estimates of distance are possible. 
In contrast to the nocturnal prosimians, the higher primates are active by day. 
They are also typically larger than the lower primates. Incidentally, even though 
man is often pictured as a weak, defenseless creature, in reality even without 



326 • EVOLUTION AND MAN 




: i b 





\ 



■■■■■ 





Fig. 33-1. Representative prosimians. (a) 
Tree shrew (Tupaia minor); (b) Mindanac 
tarsier (Tarsius carbonarins); (c) Galagc 
(Galago crassicaudatHs); (d) Aye-aye (Dam 
bentonia madagascariensis); (e, left) Mous( 
lemur (Ai/crocebus murinis). (With permis 
sion of Zoological Society of London [a, d 
and e], Walker [b] and Chicago Zoologica 
Park [c].) 



modern weapons he is a rather formidable animal, as are the orangutan, chim- 
panzee, and gorilla. In addition to improved vision, the most striking difference 
between higher and lower primates lies in the larger brain of the former, with 



HUMAN EVOLUTION • 327 

the cerebral cortex assuming ever-greater importance. The cerebrum, where the 
higher mental functions are localized, covers more and more of the brain until 
in man it virtually overlies the rest of the brain. 

The Anthropoidea have been divided into two major groups, the platyr- 
rhines of the Americas, including the New World monkeys and marmosets, and 
the Old World catarrhines, including men, apes, and the monkeys of the Old 
World tropics. The platyrrhines are flat nosed, with the nostrils widely spaced. 
In the catarrhines the nostrils are close together and point downward. 

In addition to their noses, perhaps the most striking trait of the New 
World monkeys is their prehensile tail by which most of them can hang or swing 
from branches or use as a fifth hand. The little marmosets scarcely look like 
monkeys, for they have claws rather than nails (except on the big toe) and their 
thumbs are not opposable. Furthermore, some of them have manes, and their fur 
typically has a banded pattern. 

The monkeys of the Old World lack prehensile tails and some species 
such as the baboons have become terrestrial, living in rocky, open country. As a 
group, the Cercopithecidae are more generalized in body form than the New 
World monkeys, and are not so completely adapted to arboreal life. Their hands 
look rather human, for the thumbs have good opposability. The Old and New 
World monkeys differ not only in their distribution and the traits just mentioned 
but also in such fundamental anatomical traits as dentition and structure of the 
skull. See Fig. 33-2. 

The two remaining groups of catarrhines, because of their similarities, 
have been placed in a single superfamily, the Hominoidea. These two groups are 
the anthropoid apes of the family Pongidae and the family to which man himself 
belongs, the Hominidae. The living anthropoids are the gibbons, the orangutans, 
the chimpanzees, and the gorillas. Man and the apes show many more similarities 
than man and the monkeys. Not only are they large in size and lacking a tail, 
but in many fundamental morphological and physiological traits they are much 
alike. In the details of their brain and skull, dentition, and skeleton they show 
rather close affinities. Many of these resemblances result from the adoption of an 
erect posture with the associated changes in such traits as the shape of the chest, 
the position of the abdominal organs, and the shape of the pelvis. Furthermore, 
in such matters as reproductive physiology, blood group chemistry, and even 
susceptibility to parasites, they show evidence of rather close genetic ties. The 
main differences between man and the apes are associated with their modes of 
locomotion, for the apes are essentially brachiators, swinging upright through the 
trees by their hands, while man walks erect on the ground. The gibbons, superb 
aerialists, live in the tropical forests of Southeast Asia. The orangutan also lives 
in this region but is now confined to the islands of Borneo and Sumatra. Orangs, 
like the gibbons, are completely arboreal, but since they are much larger than 
gibbons, they are comparatively slow moving and deliberate in their actions. The 



328 • EVOLUTION AND MAN 










Fig. 33-2. Representative monkeys. New 
World: (a) Humboldt's woolly monkey 
{Lagothrix lagotricha); (b) Lion-headed 
or golden marmoset (Leontocebus rosalia). 
Old World: (c) Pig-tailed macaque (Ma- 
caca nemestrina). (With permission of 
Walker [a, b] and National Zoological 
Park, Smithsonian Institution [c].) 



HUMAN EVOLUTION • 329 






"- % 




Fig. 33-3. The anthropoid apes, (a) Gibbon (Hylobates); (b) Orangutan (Pon- 
go); (c) Gorilla (Gorilla); (d) Chimpanzee (Pan). 



other two anthropoids, the gorilla and the chimpanzee, inhabit the tropical 
forests of west central Africa. However, they are not as strictly arboreal as the 
Asiatic apes, for they spend a considerable part of their time on the ground. 
Nevertheless, their body structure is still essentially that of a brachiator though 
not so completely specialized for this mode of life as the gibbon or orangutan. 
See Fig. 33-3. 

Man is not only a Primate, he is an Old World catarrhine and even 
more specifically, his anatomy shows him to be a hominoid, a member of the 



330 • EVOLUTION AND MAN 

same superfamily as the great apes. The hominid traits, which set him apart from 
the apes, are his feet and legs, which enable him to walk erect on the ground, 
with his hands free for tasks other than locomotion (see Fig. 33-4). Man's skull 




Fig. 33-4. The upstart. 

and brain also set him apart from the apes, but these are apparently differences in 
emphasis rather than in basic structure, and moreover they arose after the differ- 
ences in leg structure had evolved. Aside from his obviously larger brain, man's 
head differs from the apes' in that the face is reduced in size and has shrunk back 
under the forehead. This recession of the face appears related to the better 
balance of the skull on the spine achieved by man as compared to the apes. 
Associated with this change has been a reduction in the size of the teeth and 
jaws and the emergence of the distinctive human nose and chin (see Fig. 33-5). 



Fossil Primates 

The actual fossil record of the Primates is fragmentary and in many 
ways unsatisfactory. However, it does suffice to show that the Primates are one 
of the oldest orders of mammals, having a fossil record extending well back into 
the Mesozoic. By the Paleocene at the beginning of the Tertiary a number of 
prosimians, such as lemurs, lorises, and tarsiers, were present in relative abun- 
dance over most of the world. After flourishing during the Paleocene and 
Eocene, the prosimians vanished completelv from the Oligocene in North Amer- 
ica and Europe and were reduced in numbers in Asia and Africa. The reasons for 
their decline are not known, but the prosimian hard times coincided not only 



HUMAN EVOLUTION • 331 




Chimpanzee 




Java man 





African ape - man 
(Australopithecus) 




Neanderthal man Modern man 

Fig. 33-5. Five hominid skulls shown with that of an anthropoid ape for comparison. 



332 • EVOLUTION AND MAN 

with the rise of such mammalian groups as the carnivores and rodents but also 
with the appearance of the higher primates, fossils of which first appear in the 
early Oligocene. It seems a fairly safe assumption that competition from these 
highly successful groups played a significant role in the decline of the lower 
primates. On the island of Madagascar, which the lemurs reached but the higher 
primates did not, the lemurs continued to survive and evolve, long after they 
became extinct elsewhere in the world. 

Fossils that suggest the hominoid line leading to the apes and man also 
appear at about this time, some fifty million years ago. More is known of the 
fossil precursors of the gibbons than of the other hominoids. Propliopithecus 
from the Oligocene of Egypt some 35 million years ago, Limnopithecus from 
East Africa, in the Miocene some 10 million years later, and Pliopithecus from 
the late Miocene and early Pliocene in Europe lead quite clearly up to the 
modern gibbon, Hylobates. The gibbons thus were separated quite early from 
the other lines of hominoids. Furthermore, they were not as specialized for 
brachiation as the modern gibbons, but had more generalized limbs for climbing. 

Only in the Miocene do the forerunners of the other apes begin to ap- 
pear in the fossil genera Dryopithecus and Sivapithecus, found rather abundantly 
in Europe, Asia, and Africa. The remains consist almost exclusively of jaws and 
teeth, so that little is known about whether they were brachiating animals. How- 
ever, the teeth and jaws seem clearly to be of a type that today are found in 
modified form in the great apes (gorilla, chimpanzee, and orangutan) and in 
man. Fossils of an even earlier type of ape known as Proconsul have been found 
in relative abundance in lower Miocene deposits in East Africa. Although 
Proconsul has been thought to be a forerunner of the modern chimpanzee, in 
reality he shows differences from all other hominoids, the gibbons, the great 
apes, and man, as well as from the fossil Dryopithecus group, and he probably 
represents a separate evolutionary line. A significant point brought out by the 
fossil record of the apes is that the living populations of apes are essentially 
relict populations; the fossil apes were evidently much more widespread and 
abundant than are the living groups. 

The Fossil Record of Man 

The existing races of man are descended from other somewhat different 
populations that lived in the past. The evidence for human evolution comes from 
the fossil record. If man has evolved, then it is necessary to try to define the 
stage in his evolution when he first became human. This stage can be defined as 
having been reached when man's ancestors became intelligent enough to make 
tools. His ancestors between the ape and human levels can then be termed pre- 
human. Though arbitrary, this definition is essentially objective. The fossil 
record of man or preman is largely confined to the Pleistocene, and our knowl- 



HUMAN EVOLUTION -333 

edge, therefore, covers primarily the last million years of human evolution. Prior 
to that time there is a gap of several million years in the actual fossil record. 

The fossil evidence indicates that the prehumans must have come from 
a generalized anthropoid ape, which lived on the ground but had arboreal ances- 
tors. The major evolutionary change leading to man was the shift to bipedal 
locomotion. This change led to changes in the bones and muscles of the pelvis, 
legs, and feet, including the realignment of the big toe, and in the angle of 
attachment of the skull to the spine. The net effect enables man to keep his body 
erect, not by muscular action as the apes do, but on a bony supporting column. 
The erect posture freed the hands from use for locomotion. The ancestral 
hominids were probably omnivores who shifted toward carnivorous ways and 
developed systematic hunting habits, for the earliest known men were hunters 
using weapons to kill game. It is probable that sticks and stones were used first, 
and that this, in turn, led to weapon making. The fossils indicate that much of 
the increase in brain size came later during the Pleistocene after the shift to 
bipedal locomotion. 

Man and his tools appear first in the major warmer parts of the Old 
World — that is, in Africa and southern Eurasia — and presumably prehuman evo- 
lution occurred somewhere in this area. No living or fossil apes or prehumans 
are found either in America or in the Australian region, a fact that would also 
seem to rule out northern Eurasia as the place of man's origin, for expansion to 
North America via the Bering land bridge would then have been a simple 
matter, as it was for many other species. The prehumans presumably evolved in 
open country where running on two legs was an advantage to incipient hunters. 
Such a setting points to Africa, where there was much open country and much 
game to run after. Whatever the place, they then dispersed, as other dominant 
successful groups of animals have done, in a complex fashion over the warmer 
parts of the Old World during the Pliocene. The first known tools of worked 
stone, from Olduvai Gorge in Tanganyika, are estimated to have been made 
1,750,000 years ago, or only 70,000 generations ago. 

During the previous 70 million years or so in the Tertiary the climate 
of the earth was rather warm and stable. With the beginning of the Pleistocene 
about a million years ago, the earth's climate became changeable and a period of 
cooling was followed by four major ice ages, with intervening warmer periods. 
Four times, tremendous continental glaciers pushed their way down into the 
more temperate regions, covering major portions of Europe, North America, and 
parts of Asia. The glacial stages were followed by warmer interglacial stages 
during which the weather became even warmer than at present. Thus the Pleisto- 
cene was a time of fluctuating, unsettled climatic conditions. The stages have 
been most carefully studied in Europe and North America, and their names and 
approximate durations are shown in Table 33-2 together with relevant informa- 
tion about fossil man. 



334 • EVOLUTION AND MAN 



TABLE 3 3-2. Fossil Men 



Glacial and interglacial 

stages 
(estimated years ago) 



Cultural period 



Fossil hominids 



Postglacial 



10,000 

IV Wisconsin 

or 
Wiirm Glacial 



80,000 



Third Interglacial 



150,000 

III Illinoian 
or 

Riss Glacial 
225,000 

Second or 

"Great" Interglacial 

350,000 

II Kansan or 
Mindel Glacial 
425,000 



First Interglacial 



500,000 

I Nebraskan or 

Giinz Glacial 
625,000 

Possible earlier glacials 

1,750,000 



Iron 
Bronze 
Neolithic 
Mesolithic 



Upper Paleolithic 



Middle Paleolithic 



Lower Paleolithic 



Cro-Magnon 



Florisbad 
Combe-Capelle 

Neanderthal 
Solo 

Rhodesian 
Mount Carmel 

Neanderthal 
Fontechevade 

Kanjera 
Rabat-Casablanca 



Steinheim 
Swanscombe 

Peking 

Java 

Ternifine 

Heidelberg 

Telanthropus 

Paranthropus 

Java 

Meganthropus 

Australopithecus 

Kanam (?) 

Australopithecus 

Zinjanthropus 

Unnamed Olduvai hominid 



The tendency to assign each newly discovered fossil member of the 
Hominidae to a separate genus has led to a confusing welter of names. Rather 
than bringing out the similarities and differences among these fossils, the system 
has obscured their relationships. Use of the same taxonomic criteria for hominids 



HUMAN EVOLUTION -335 

as for other groups would considerably reduce the number of genera and species. 

Although the fossil record of man is largely confined to the last million 
years (the Pleistocene) , a recent restudy of a fossil known as Oreopithecus from 
the lower Pliocene in Italy some ten million years ago may carry our knowledge 
further back in time. When found a century ago, he was assigned to the Old 
World monkeys or Cercopithecidae and was more or less ignored. However, 
further finds and restudy in the light of modern knowledge have shown that 
Oreopithecus clearly is neither a monkey nor a Proconsul nor a Dryopithecus. If 
he must be assigned to one of the three hominoid groups (gibbons, great apes, 
hominids), he comes closer to being a hominid than anything else. This is not to 
say that he is necessarily a direct ancestor of man, but rather that he probably 
belonged some ten million years ago to the same group of related species that 
included the ancestors of man. 

The most primitive kind of fossils that are clearly those of Hominidae 
come from deposits in South Africa. Dr. Raymond Dart, the anatomist who 
made the original discovery, called his find Australopithecus and pointed out its 
human characteristics. This conclusion was at first widely doubted and challenged 
by many of the recognized authorities. However, further discoveries by Broom, 
Robinson, Leakey, and others have shown almost beyond question that Australo- 
pithecus was indeed an early hominid and not simply an anthropoid ape with 
some slightly human traits. These additional fossils, as is customary, have been 
given separate generic names (for example, Paranthropus, Plesianthropus, Tel- 
anthropus, and Zinjanthropus), but all are enough alike (except perhaps Telan- 
thropus) to be put in the same subfamily, the Australopithecinae. Further mate- 
rial and additional study may in time lead to taxonomic revision toward greater 
simplicity. However, the fossils fall into two main groups, typified by Australo- 
pithecus and Paranthropus. The Australopithecus type was rather small, probably 
weighing no more than 50 or 60 pounds; Paranthropus was considerably larger 
and heavier. The most significant features of these australopithecine "ape-men" 
were their rather small brains, with a cranial capacity of about 600 cc — not much 
greater than that of a gorilla or a chimpanzee — associated with pelvic and leg 
bones very similar to those of modern man. Thus, it seems that erect bipedal 
locomotion on the ground — in other words, walking erect — evolved first in the 
human line and that the increase in size and capability of the human brain 
evolved later. This conclusion is contrary' to what was long believed to be the 
case, that man was an intelligent ape who climbed down from the trees to take 
up his abode on the ground. Furthermore, the "ape-men" had relatively massive 
jaws, but the details of the jaws and of the dentition were fundamentally human 
and not apelike at all, and the skull, despite the small size of the brain, was of 
the human pattern. Finally, evidence has been accumulating, climaxed by the 
recent discovery by the Leakeys of a new fossil hominid that they called Zinjan- 
thropus and another hominid as yet unnamed, that the Australopithecinae were 



336 • EVOLUTION AND MAN 

already capable of using and even fabricating simple stone and possibly bone 
tools some 1,750,000 years ago. This discovery has again required a considerable 
revision in our thinking about the course of human evolution, If we are to define 
as "human" those hominids who could make tools, then the terms "man-apes" 
and even "ape-men" seem to be inappropriate for the Australopithecinae. Knowl- 
edge of this group has greatly increased our information about the course of evo- 
lution in the Hominidae and makes even more urgent the need to find additional 
early hominid remains. 

A more advanced stage of human evolution is represented by the fossils 
first found by Dubois and known as the Java man. They illustrate very nicely the 
taxonomic problems in paleoanthropology. Dubois christened his find Pithecan- 
thropus erectus and placed him in a new family, Pithecanthropidae, between the 
Pongidae and the Hominidae. The quite similar Peking man was originally 
placed in a new genus and species, Sinanthropus pekinensis. However, the simi- 
larities have led some authorities to regard them as two species in the same 
genus, Pithecanthropus erectus and P. pekinensis, and Mayr has argued that by 
the usual taxonomic criteria they are merely different races of the same species 
and that this species is sufficiently like modern man to be placed in the same 
genus, Homo. Thus Java man would become Homo erectus erectus and Peking 
man H. e. pekinensis. These differences over nomenclature may seem to be 
trivial, but the implications of each system are quite different. It now appears 
certain that the two finds belong to the Hominidae rather than to a separate 
family and that they belong together in the same genus. Since their cranial capac- 
ities were quite different, they differed more than the living human races, and 
thus the best course may be to take the middle ground and consider them as 
separate species within the same genus, Pithecanthropus. The Java and Peking 
men, living perhaps half a million years ago, were hunters with stone tools who 
lived in caves and used fire. They had thick skulls with heavy brow ridges, a 
prognathous profile with large teeth but no chin, and a cranial capacity of ap- 
proximately 750 to 900 cc in Java man and 900 to 1200 cc in Peking man. The 
rest of their skeleton did not differ from that of modern man. One habit of these 
early humans is clearly recorded. They picked each others' brains and tossed the 
skulls aside in their caves, there to be discovered thousands of years later as evi- 
dence of their cannibalism. 

Another stage in the evolution of man represented by abundant fossils 
is known as Neanderthal man, after the valley in Germany where the first care- 
fully studied fossils of this type were discovered in 1856. Numerous fossils of 
Neanderthal men (and here we are clearly dealing with members of the genus 
Homo) were found in North Africa, in western Asia, and over most of Europe 
except Britain and the northern regions. They persisted for about a hun- 
dred thousand years, first appearing in the Third or Last Interglacial, and 
being found in even greater numbers in the first part of the Fourth or Wisconsin 



HUMAN EVOLUTION • 337 

Glaciation. Then, quite suddenly (a matter of centuries, actually) they dis- 
appeared, being replaced throughout their range by men like ourselves. The 
average size of their brains (about 1450 cc) was somewhat larger than the 
average for the brain of living men (about 1350 cc). The skull was thick walled 
and low and bulged at the sides, with the rear drawn out into a projecting 
occipital region, which was marked by a ridge for the attachment of massive 
neck muscles. The retreating forehead sloped back from heavy brow ridges, and 
the face and teeth were relatively large. The lower jaw was heavy, but lacked the 
protruding chin of modern man. The rest of the skeleton indicates that Neander- 
thal men were only about five feet tall but of an exceptionally powerful, muscu- 



^^ti,:\s: 




ANCIENT MEN 




Sterkfontein 




Fig. 33-6. Fossil hominid skulls of the Pleistocene epoch. Relative age is shown 
by position; the names indicate the initial place of discovery. The general trends 
in hominid evolution can be observed from the Australopithecinae at the bottom 
through Pithecanthropus (Java, Peking, and Solo men) and Neanderthal (in- 
cluding Shanidar) to Homo sapiens (Cro-Magnon and Combe-Capelle) at the 
top. The Mount Carmel skull shows traits of both Neanderthal and modern 
man. (Redrawn after Washburn.) 



338 • EVOLUTION AND MAN 

lar build. Because of these rather well-defined differences from living men, the 
Neanderthals have been placed in a separate species of the genus Homo, H. 
neanderthalensis, though they have also been called a race of Homo sapiens. 

Men of our own species, Homo sapiens, do not appear in the fossil record 
until about 35,000 B.C. These tall and well-built men, of the so-called Cro- 
Magnon type, had a distinct bony chin on the front of the jaw, a high-domed, 
thin-walled skull, and greatly reduced brow ridges, and are indistinguishable 
from modern men. These are the people who, in a relatively short time, com- 
pletely replaced the Neanderthal type. However, just where this modern type of 
man came from and who his immediate predecessors were are far from clear. At 
the present time only a single genus of the family Hominidae and a single 
species within that genus, Homo sapiens, exists on the earth. All mankind be- 
longs to this one species. Human fossils of types clearly belonging to the genus 
Homo have been found as far back as the late Middle Pleistocene, but the record 
is quite fragmentary and incomplete, and the relations of these fragments to one 
another and to modern man are obscure. See Fig. 33-6. 

The Origin of Modern Man 

The theories of the origin of the living races of man range from a 
simple straight-line evolution from Australopithecus -» Pithecanthropus -» 
Neanderthal — » Modern man, to a polyphyletic scheme in which each living 
human race is derived from a different series of fossil ancestors. Although it is 
a fairly safe assumption that neither of these theories is correct, the available 
evidence is insufficient to establish man's lineage. Many of the known fossil 
hominids are listed in Table 33-2 where it can be seen that fossils of rather 
different types (for example, Paranthropus and Pithecanthropus, and later, 
Pithecanthropus and Homo, represented by the Steinheim and Swanscombe 
skulls) were contemporary. Such information suggests that in the Hominidae as 
in other groups, evolution gave rise to several diverging lines, many of which 
became extinct while others eventually gave rise to new species. Though only one 
species, Homo sapiens, now exists, it has already diverged to some extent in the 
formation of the various human racial groups. The details of the skull and facial 
skeleton of Cro-Magnon man show that he was a member of the Caucasoid 
racial group. However, it does not necessarily follow that the other races are 
derived from the white race. Indeed it is unlikely that any existing race was an- 
cestral to the others. Rather, it is probable that all of the living races have 
diverged somewhat from the ancestral population of Homo sapiens from which 
they all are descended. What is suggested is that, even though the exact time of 
origin of Homo sapiens is not yet known, the Caucasoid Cro-Magnon men show 
that divergence toward modern races had already occurred and that modern man 
must have originated at some time prior to 35,000 B.C. 



HUMAN EVOLUTION • 339 

The relationship between Neanderthal and modern men has constituted 
somewhat of a puzzle. Similarly, while it is reasonably certain that the modern 
Homo sapiens type of human replaced Neanderthal man throughout his range 
in a rather brief interval, the cause of his extinction remains unknown. Direct 
combat leading to extermination of the Neanderthals may be the answer, but it is 
not the only one possible, for more subtle forms of competition — for game or 
caves, for example — could have had the same ultimate effect. It has even been 
suggested that where the two groups met, they interbred, and that the Neander- 
thals were absorbed rather than eliminated. For the most part, the evidence does 
not support this idea. However, on the eastern shore of the Mediterranean in 
caves on the slopes of Mount Carmel in Palestine have been found skeletons 
that show a strange mixture of Neanderthal and sapiens traits. One interpretation 
of this material is that it is the result of hybridization between the two groups. 
Although it is true that the Near East has long been the crossroads of the world 
for mankind, and from that standpoint this interpretation seems reasonable, 
nevertheless other explanations have also been advanced; for example, that these 
people represent the last stage in a transition from Neanderthal to sapiens. How- 
ever, other evidence makes this hypothesis difficult to uphold. For one thing, 
skulls quite different from Neanderthal and tending toward sapiens are already 
known from the late Middle Pleistocene (Steinheim and Swanscombe), and 
definitely sap i ens-like skulls (Fontechevade and Kan j era) are found in the early 
Upper Pleistocene, well before the time of the Mount Carmel material and even 
before the time of the Neanderthals themselves. Furthermore, the early Neander- 
thal men from the Third Interglacial were not as extreme in their distinguishing 
features as those from the Fourth Glacial. What this evidence suggests is that, 
rather than being the direct ancestors of modern man, the Neanderthalians were 
a divergent group, which perhaps became especially well adapted to survive the 
rigorous climate of the last ice age, but were eventually overrun and supplanted 
by a new and even more successful human type. That this explanation may be 
correct is suggested by the fact that the new people apparently brought with 
them a new and more advanced culture. The Mousterian tools associated with 
Neanderthal man were replaced by the more refined Aurignacian stone tools of 
the Upper Paleolithic men. The Neanderthalians had developed a distinctive cul- 
ture of their own. There is evidence of religious concepts in their ceremonial 
burial of the dead and in their worship of cave bears, the fearsome enemies with 
whom they fought for the caves essential to their survival during the last ice age. 
They were skilled hunters, able to take game as large as the mammoth and the 
woolly rhinoceros. However, the culture of their Cro-Magnon successors was 
considerably more advanced, marked not only by new and improved stone tools 
and weapons, but by evidence of great hunting skill and the notably graceful art 
in their caves. 

At present we have a glimpse here and there of stages in human evolu- 



340 • EVOLUTION AND MAN 

tion during the past million years sufficient to show that evolution in the 
Hominidae has progressed quite rapidly during this time. The major adaptive 
shift that led to the separation of the hominids from the apes was the change in 
the lower limbs and pelvis, which permitted walking erect. This shift was es- 
sentially complete in the Australopithecinae, and the evolution of a progressively 
larger brain was a subsequent development Fossil hominids of diverse kinds are 
widely scattered over the Old World, signs of a successful, expanding group, 
but the place of origin of the Hominidae is as yet unknown. Although indica- 
tions at present point to Africa, this may be simply because the record is more 
complete from that area. Although Pithecanthropus (Java and Peking men) may 
be regarded as a stage intermediate between the Australopithecinae and modern 
man, they may or may not be in the direct line of descent. Of the other fossil 
men available, many are poorly known, either because only a few fragments have 
been found or because the material has not been adequately dated. Although the 
relationship between Neanderthal man and modern man, as represented by the 
Cro-Magnon type that so dramatically superseded -the Neanderthalians, is still in 
doubt, the best guess is that both were derived from one of the earlier types of 
Homo now known only from a few scattered skeletal remains. 

Therefore, the fossil record of man, incomplete and fragmentary though 
it is, is sufficient to show that in the past somewhat different human types did 
exist from which modern man has descended; it is not complete enough to show 
exactly what the course of evolution leading to Homo sapiens has been. New 
human fossils are being found at an accelerating pace, however, and there is 
reason to hope that in time some of the basic questions about man's origins can 
be answered more fully than at present. The picture may appear to become more 
confusing before it is clarified, for it seems unlikely, since isolation exists be- 
tween different human populations, that evolution leading to man would follow 
a simple, straight-line pattern any more than it would in any other group. 

We cannot leave our discussion of man's fossil record without some 
mention of one of the most successful hoaxes in history, the Piltdown man, 
dignified by the scientific name, Eoanthropus dawsoni. Fragments were reported 
from a gravel pit at Piltdown in Sussex, England, between 1908 and 1915. 
When reconstructed, they took the form of a brain case much like that of modern 
man, though thicker, and a lower jaw like that of a large ape. This find fulfilled 
the then-current concept of what the "missing link" between man and the apes 
would be like. Accepted as authentic, studied and puzzled over by experts, the 
Piltdown man went unexposed for over forty years. The subsequent finds of 
fossil hominids, especially the australopithecines, made an ever-greater anomaly 
of Eoanthropus, for they all agreed in having hominid jaws and dentition asso- 
ciated with a skull rather like an ape's instead of the reverse. Eventually with the 
aid of modern techniques, the Piltdown man was shown beyond question to be 
a clever fraud concocted from a human skull and the carefully doctored 



HUMAN EVOLUTION • 34 1 

jaw and teeth of an orangutan. Even the tools and animal fossil bones found at 
the same site turned out to have been planted. Surely this was one of the most 
successful practical jokes in history, but modern methods of dating and analysis, 
if not the lesson learned here, make it very improbable that anthropologists will 
ever again be fooled in this way. 

Man, a Polytypic Species 

Although it has been argued that there are several living human species, 
it is clear that if the same taxonomic criteria are applied to man as have been 
applied to other species, there is but one human species living at the present 
time. This species, Homo sapiens, is polymorphic, for every human population 
manifests considerable variability, a fact easily confirmed by a quick glance at 
your friends and neighbors. It is also polytypic, for many geographic subspecies 
have been distinguished and named. They are not separate species, however, 
because the different races can and do interbreed. Probably the only racial cross 
that has not occurred is between Eskimos and African Bushmen. Furthermore, it 
is not possible to draw sharp, distinct lines of demarcation between human racial 
groups since one race usually blends into another in the zone of contact. The 
different human races differ from each other in the incidence of certain of their 
genes, and this is the basic distinction between races. While all living men must 
share fundamentally similar genotypes that cause them to develop into members 
of Homo sapiens, different human populations have diverged from one another 
to some extent. Human populations, past and present, are subject to the effects 
of mutation, natural selection, random genetic drift, and gene flow just as are 
other species. 

Far too little is known about adaptive values in man, and man's present 
high mobility tends to obscure still further his adaptations to local conditions, 
but the indications are that the different human races are adapted to their imme- 
diate environments. The relation between degree of skin pigmentation and 
amount of exposure to the sun is a familiar example, but perhaps a somewhat 
shaky one since the skin has functions other than to serve as a filter for the ultra- 
violet light needed to form vitamin D in the body. Body form shows an even 
closer relation to climate than does skin color. The surface-to-volume ratio is 
maximized for more efficient heat dissipation in the lanky desert Arabs and 
Nilotic Negroes living under the searing tropical sun, but is minimized in the 
roly-poly Eskimos. The nasal cavities of Eskimos and north Europeans have also 
been shown to be better suited for warming and moistening cold, dry air than 
those of peoples living under milder climates. In fact, the entire Mongoloid face 
is thought to be adapted for life in a cold climate, for the nose is reduced and 
the entire face is flattened out and padded with fat, and the eyes are protected by 
the so-called Mongoloid fold. The steatopygia or fat on the buttocks of African 



342 • EVOLUTION AND MAN 

Bushwomen is another trait often cited as adaptive, for they store fat there in 
remarkable quantities. Although it has been suggested that steatopygia is func- 
tionally analagous to a camel's hump — an energy reserve that does not limit heat 
dissipation — this explanation fails to explain why the trait is absent in the male. 
It may be related to food storage for sustaining pregnancy, but sexual selection 
may also play a role, for the trait is said to be much admired by the men. 

The sickle cell gene discussed earlier is one of the best-understood cases 
of adaptation in man. In regions where malaria is prevalent, the heterozygote for 
this gene is better adapted to survive than either homozygote; for one individual 
(Hb s /Hb s ) is done in by his harmful genes, whereas the other (Hb a /Hb a ) is 
apt to be carried off by malaria. Hence a balanced polymorphism due to heterosis 
exists, and in some regions over 40 percent of the population may carry the sickle 
cell gene, a high frequency out of all proportion to what might be expected of a 
gene with such drastic effects in the homozygous condition. Though this gene is 
most common in Negro Africa where its highest frequencies coincide roughly 
with the highest incidence of malaria, it is not restricted to this region or to this 
race, for it has also been found in malarial regions of India, Greece, Italy, 
Turkey, and Arabia. The most reasonable explanation for the distribution of the 
sickle cell gene is that it arose by mutation, probably among the Negroes in 
Africa, and has been introduced into other regions and races by gene flow 
through occasional matings between the Mediterranean peoples and Negro car- 
riers. Once established, its frequency increased owing to its selective advantage 
in malarial areas. It is not, however, found in all regions of the world where 
malaria exists, presumably because it never got there either by mutation or by 
migration. However, other genes similar in function but distinct from the sick- 
ling gene have been discovered. As a final footnote to this story, the primary 
effect of the sickle cell gene, so far-reaching in its ultimate effects, has been 
shown to be merely the substitution in normal adult hemoglobin of a single 
amino acid, valine, for another, glutamic acid, in one of the peptides making up 
the hemoglobin protein molecule. 

Many questions remain to be answered. What adaptive value, if any, is 
there in the different eye colors in man or in the different color and shape of 
human hair? Why do some races have much more body hair than others? What 
factors are responsible for the development of the pygmy tribes ? The list could 
be considerably extended, but the answers in nearly all cases, are unknown or at 
best merely informed guesses. In principle, we know that the differences must 
have arisen through the combination of directive and chance elements that govern 
the course of evolution within breeding populations (mutation, selection, genetic 
drift, and migration) ; in detail, however, our knowledge of the origin and 
function of the traits that distinguish one human race from another is quite 
sketchy. Many traits seem unlikely to confer any adaptive value, but even this 
assumption cannot be taken for granted. The different blood groups of the ABO 



HUMAN EVOLUTION • 343 

system were long cited as traits in man governed by neutral genes, but it now 
appears, for example, that stomach cancer is somewhat more likely to develop in 
people of type A, and people of type O are somewhat more susceptible to duo- 
denal ulcers, and thus this example must be discarded. 

The Races of Man 

There is not and probably cannot be any general agreement on the 
number of distinct human races. More than thirty have been distinguished. 
However, at least six rather distinct racial groups can be recognized as follows 
(see also Fig. 33-7) : 

Race Distribution before 1492 

1. Negroid Widely scattered. Tropical Africa and Old World tropics — 

India, Andaman Islands, Philippines, Queensland, New 
Guinea, islands east to Fiji and southeast to New Caledonia 

2. Caucasoid North of tropics in North Africa, Europe, and Western 

Asia, southeast into tropics in India 

3. Mongoloid North and East Asia, south into Sunda Islands, North and 

South America 

4. Bushmen South Africa 

5. Australoid Australia 

6. Polynesian Remote Central Pacific islands from New Zealand to Hawaii 

In terms of numbers and widespread distribution, the Negroid, Cau- 
casoid, and Mongoloid groups are the three major human races at the present 
time. Negroids are usually dark skinned with black woolly hair, broad, flat noses, 
and thick lips. Caucasoids generally have rather light skin, long, narrow noses, 
and relatively straight hair. The hair of Mongoloids is straight and black, an eye 
fold is common, and the face is flattened with high cheekbones. As soon as these 
descriptions have been given, they must immediately be qualified because there 
is a great deal of variation within each group. All the races vary considerably in 
skin color, for example. The Caucasoid or so-called "white" race varies all the 
way from the blond, blue-eyed Scandinavian to the dark-eyed, dark-skinned 
Hindu of India. The Mongoloid group includes not only the "yellow" skinned 
Asians and Eskimos but the American "redskin." In size, Negroids vary from 
the tall Watussi (Batutsi) tribe whose members approach seven feet, to the 
Pygmies whose males average under five feet in height. Furthermore, the con- 
tacts between Negroids and Caucasoids in northern Africa and between Mongo- 
loids and both Negroids and Caucasoids in the Orient have effectively blurred 
any distinctions between the races. In fact, the concept of "pure races," the idea 
that Homo sapiens in prehistoric times consisted of a group of separate, distinct 



344 • EVOLUTION AND MAN 

racial groups whose differences are gradually being eroded away by the coming 
of civilization is so improbable as to be relegated to the realm of myths. Al- 
though local populations in the past undoubtedly were somewhat more isolated 
than at present, variability within populations and gene flow between popula- 
tions, then as now, would have prevented the development of a "pure race." 





f":'S." &£? 




Fig. 33-7. Representatives of major human races, (a) Mongoloid: Alaskan 
Eskimo woman; {b) Negroid: South African Bantu woman; (c) Bushmen: 
Hottentot woman with steatopygia; (d) Australoid: Girl from Northern Aus- 
tralia; (e) Polynesian: Maori woman; (/) Caucasoid: United States. (Courtesy 
of Peabody Museum, Harvard University.) 



HUMAN EVOLUTION • 345 

The Nazi concept of a pure Nordic race as the original Europeans and the 
builders of modern civilization simply does not stand up in the light of our 
knowledge of modern genetics and anthropology. Human populations have 
never been static entities. They have adapted to changing physical and biological 
conditions. The net result of natural selection, hybridization, mutation, and 
genetic drift has been an ever-shifting pattern in human breeding populations. 
Some have disappeared, either completely or by absorption into others by inter- 
marriage, while distinctive new populations have appeared. In the past, isolation 
by distance appears to have been the significant factor that permitted the differ- 
entiation of Homo sapiens into recognizably different racial groups. At present, 
isolation is breaking down, and new and different human types are arising as the 
result of hybridization. The mestizos of Latin America, a mixture of European, 
Indian, and some Negro ancestry, and the inhabitants of Pitcairn Island, de- 
scended from Europeans and Polynesians, represent examples of this sort. 

The remaining three races of man mentioned above, the Polynesians, 
the Australoids, and the African Bushmen and their Hottentot relatives, are in a 
sense peripheral human groups. The Bushmen are found only in southern Africa, 
the Australoids in the Australian region, and the Polynesians on the islands in 
the far reaches of the Pacific. This distribution pattern calls to mind the distribu- 
tion of relict populations in our discussion of biogeography. Although the 
analogy may hold with respect to the Bushmen and the Australoids, who appear 
to have occupied their present territory for some time, the Polynesians seem to 
have reached their island realm only quite recently. The Polynesians, despite the 
arguments based on the Kon-Tiki voyage, appear very definitely to have originated 
in Asia and not in South America. 

Cultural Evolution 

In addition to his own fossil remains, early man left behind him another 
type of record, a record of his culture. These cultures are known as the Paleo- 
lithic (or Old Stone Age) , Mesolithic, Neolithic, and the Bronze and Iron Ages. 
These broad stages are used to indicate the cultural status achieved by a people 
and do not necessarily indicate absolute divisions of time, for some peoples are 
just now emerging from the Neolithic, a stage through which others passed 
several thousand years ago. The nature of past human cultures is inferred from 
the form of their tools and weapons and other implements (see Fig. 33-8). The 
Stone Age ranged in time from the Pliocene up until a few thousand years ago. 
The Neolithic, which marked the invention of agriculture, began only about 
ten thousand years ago; man, therefore, has been a hunter and gatherer of wild 
plant food for all but about 1 percent of his known existence. Even a high level 
of skill in hunting was reached only about thirty-five or forty thousand years ago 
in the Upper Paleolithic. Hence early cultures changed only very slowly and 



346 • EVOLUTION AND MAN 




UPPER PALEOLITHIC 



\ V 



<35^ 




BLADE TOOLS 




MOUSTERIAN 



S\ .4 







111 



FLAKE TOOLS 




ACHEULEAN (MIDDLE) 




ABBEVILLEAN 

CORE TOOLS 



PEBBLE TOOLS 



Fig. 33-8. The tool traditions of Europe form the basis for classifying 
Paleolithic cultures. The tools are arranged according to age, with the 
oldest at the bottom. Two views of each tool are given except for the 
blade tools, which are shown in three views. Tool traditions have been 
named for the site of discovery. (With permission of Washburn.) 



HUMAN EVOLUTION • 347 

persisted for long periods, but the pace of cultural changes has been ever 
increasing. 

The stone implements were made by chipping and flaking pieces from 
a flint core to fashion the desired tool or weapon, and in some cultures the flakes 
were also used for a variety of smaller implements. Since the tools were fash- 
ioned with different techniques and varying degrees of skill and complexity, it 
has been possible to recognize a number of different tool traditions, and these 
have usually been named after the place where they were first discovered. Since 
tool making underwent gradual change and improvement, the evolution concept 
has been applied to the succession of tool traditions. Although perhaps useful 
for descriptive purposes, such application holds certain pitfalls, for the evolution 
of tools is not biological evolution and does not necessarily parallel the biological 
evolution that must have been going on in man at the same time. Furthermore, 
two quite different types of men could learn to fashion the same type of tool. 
Hence, a new type of inheritance, cultural inheritance, appears. Cultural patterns 
and traditions could not only be passed from one generation to its successors, but 
could be imitated and widely and rapidly disseminated without the necessity for 
any sort of biological continuity. Thus the attempts to link a particular tool tradi- 
tion with a particular kind of fossil man are really valid only when there is posi- 
tive evidence of association. 

The earliest recognizable tools were associated with the Villafranchian 
fauna. The recent discovery of Zinjanthropus in association with stone tools 
of the pre-hand-axe Oldowan type shows that even the australopithecines had a 
true stone culture. The Abbevillian and Chellean hand-axe cultures were suc- 
ceeded by the advanced Acheulean type of hand axe. The Clactonian flake in- 
dustry, contemporaneous with these early hand-axe based cultures, was followed 
by the Mousterian-Levalloisian type of stone implements, which were more elab- 
orate and carefully made than anything that preceded them. The Mousterian 
stone tools seem to have been fashioned by Neanderthal men, for Neanderthal 
skeletal remains have often been found with Mousterian weapons and tools. The 
rapid replacement of H. neanderthalensis by H. sapiens in Europe coincided with 
the appearance of Aurignacian implements, followed in a relatively short period 
by the Solutrean and Magdalenian types. It seems safe to assume that this new 
kind of man had developed new capabilities in fashioning his tools and weapons, 
for they were of a refinement and variety not previously seen. In addition to 
stone he used materials such as bone, horn, and ivory to fashion ornaments as 
well as weapons and tools. The Paleolithic, then, endured for by far the greater 
part of man's existence and was marked by gradual but accelerating advances in 
his ability to fashion stones and other materials to his own uses. The conclusion 
is difficult to avoid that the advances were so slow at first because the earlier 
species of men were of a lower order of intelligence than the men who followed 
them. 



348 • EVOLUTION AND MAN 

With the passing of the last ice age about 10,000 years ago a new 
phase of culture, the Mesolithic, appeared. These people both hunted and fished, 
for not only did they make bows and arrows, but they fashioned nets and canoes 
and lived on fish and shellfish as well as on game. 

The Neolithic is marked by the appearance of ground and polished 
stone tools, and by pottery and weaving, but the real significance of the New 
Stone Age lay in the invention of agriculture. It was the domestication of plants 
and animals that permitted man to give up his essentially nomadic existence and 
to settle down in relatively permanent communities. Agriculture can support 
greater numbers of people than a hunting and gathering culture. Only with this 
advance did modern civilization become possible. The oldest known ground 
stone tools, cultivated plants, and domesticated animals (except for the dog) 
come from southwestern Asia and are less than ten thousand years old. This 
period seems even shorter when it is realized that agriculture was invented less 
than 400 generations ago. Agriculture apparently arose independently in at least 
three separate places. In southwestern Asia it was based on wheat, in south- 
eastern Asia on rice, and in the Americas on maize. The stone implements of the 
Neolithic were soon augmented by implements made of new materials, and the 
Bronze Age, which spread from the Near East, was soon followed by the Iron 
Age. These early civilizations bring us up to the beginnings of recorded history. 

With the development of civilization and culture, man has become a 
biologically dominant species that has expanded its range to the farthest corners 
of the earth and greatly increased in numbers. He is now cosmopolitan, the 
dominant mammalian species in all parts of the world, who has no reason to 
fear any competing species or predators, so complete is his domination by means 
of his weapons. Furthermore, he has gained mastery over most of his parasites 
and has remodeled his environment, using other species for his purposes. All of 
these developments became possible with the evolution of the human brain, the 
source of man's adaptive advantage over all other species. Human evolution has 
reached a new plateau, for superimposed on the biological evolution that still 
continues in man is cultural evolution. This new facet in evolution, the trans- 
mission of knowledge through culture, has opened up new vistas. Not only has 
he controlled the evolution of other species as he has modified domesticated 
plants and animals better to serve his needs, but he now has sufficient knowledge 
to control the course of his own evolution. Human cultural and biological evolu- 
tion are going to continue in any event. The fundamental question is whether 
man has the wisdom to guide his own future. 



SUGGESTED READING 

Boule, M., and H. V. Vallois, 1957. Fossil men. London: Thames and Hudson. 
Boyd, W. C, 1950. Genetics and the races of man. Boston: Little, Brown. 



HUMAN EVOLUTION • 349 

Clark, W. E. L., 1955. The fossil evidence for human evolution. Chicago: University 

of Chicago Press. 
, 1957. History of the primates. Chicago: University of Chicago Press, 

Phoenix Books. 
, 1959. "The crucial evidence for human evolution," Proc. Amer. Philosoph- 
ical Society, 703(2) :159-172. 

, 1959. The antecedents of man. Edinburgh: Edinburgh University Press. 

Coon, C. S., 1954. The story of man. New York: Knopf. 

Dart, R. A., and D. Craig, 1959. Adventures with the missing link. New York: 

Harper. 
Demerec, M., ed., 1950. "Origin and evolution of man," Cold Spring Harbor Symp. 

Quant. Biol., 15. New York: Long Island Biological Assoc. 
Evolution and anthropology: a centennial appraisal, 1959. Washington, D. C: 

Anthropol. Society. 
Howells, W., 1959. Mankind in the making. Garden City, N. Y.: Doubleday. 
Kluckhohn, C, 1949. Mirror for man. New York: McGraw-Hill. (Also Premier 

Reprint, 1957.) 
Tax, S., ed., I960. Evolution after Darwin, Vol. 2, The evolution of man. Chicago: 

University of Chicago Press. 
"The human species," Scientific American, 203(3) Sept. I960. 
Weiner, J. S., 1955. The Piltdown forgery. New York: Oxford University Press. 



CHAPTER 



34 

Radiation, Genetics, and Man 



One has only to look at his friends and relations to get 
some idea of the variation that exists in a natural population. 
Some of this variation, of course, is of environmental origin. The 
genetic portion is due either to the recombination and interaction 
of existing genes or to new mutations. Since existing genes at 
some time in the past also arose through mutation, mutation 
looms large as a source of variation. Mutations have been denned 
as self-duplicating changes in the hereditary material. In a broad 
sense, they include submicroscopic point mutations and the micro- 
scopically detectable rearrangements following chromosome break- 
age. The "spontaneous" mutations may be due to the natural or 
background radiation coming from radioactive minerals and cos- 
mic rays. Background radiation alone is insufficient to account for 
all "spontaneous" mutations, but a variety of chemical mutagens 
has been discovered, and these plus the effects of temperature and 
the mutation rate genes mentioned previously undoubtedly play 
a role in the induction of naturally occurring mutations. 



The Frequency of Harmful Genes 

The great majority of "spontaneous" point mutations are 
deleterious. It has been estimated that at a maximum only 1 in 
1000 is beneficial under existing conditions. The reasons for this 
fact are fairly simple. Existing genes are the product of prior 
evolution and, since they have survived the winnowing action of 
natural selection, they give rise to well-adapted organisms. Hence, 



350 



RADIATION, GENETICS, AND MAN • 351 

any change in an existing gene is far more likely to impair its function 
than to improve it. Most genes appear to be concerned with the presence 
and specificity of enzymes, and mutations, in disrupting the metabolic pat- 
tern, are generally harmful. For every lethal mutation, it is estimated 
that four detrimental mutations, reducing viability at least 10 percent, occur. 
Since this estimate is based on radiation-induced mutations in Drosophila 
(Fig. 34-1), the proportion of detrimentals among spontaneous mutants may 
actually be higher than four to one. 

Most new mutations are recessive. In other words, the normal gene is 
effective in a single dose in masking or covering up the effects of the deleterious 
or lethal mutant. Less than 1 in 100 mutants is fully dominant. Therefore, 




DOSE IN rx 10* 



Fig. 34-1. Linear relation between radiation dose and mutation 
rate for sex-linked lethals in Drosophila. (With permission of 

Begg.) 



352 • EVOLUTION AND MAN 

contrary to a widespread belief, mutation does not lead at once to a host of 
monsters in the next generation. The rare dominants are rapidly eliminated by 
natural selection, dominant lethals disappearing in the first generation. The re- 
cessives are added to the gene pool of the population. They will produce maxi- 
mum damage only when present in double dose, which may not occur for many 
generations. However, the recessive mutants are generally not completely reces- 
sive, for two doses of the normal gene ordinarily are better than a single dose 
plus the mutant and hence harmful mutations can cause damage even when 
heterozygous. This damage may be very difficult to detect since it is on the order 
of a 2 to 4 percent reduction in fecundity, fertility, viability, or longevity, with 
no obvious visible defects. Thus, a gene mildly deleterious in single dose may 
eventually do as much harm as a grossly harmful one, for it persists longer and 
has a chance to cause impairment to more individuals. Eventually it will lead to 
the extinction or "genetic death" of the line of descent carrying it, and this will 
usually happen before it becomes homozygous. 

At the present time 4 to 5 percent of the children born alive in the 
United States are in some way defective. This startling statistic may at first glance 
seem unreasonably high, but it includes not only congenital malformations but 
mental deficiency and epilepsy, and defects of vision or hearing and of the 
gastrointestinal, genitourinary, neuromuscular, hematological, and endocrine sys- 
tems. When it is realized that estimates of the frequency of mental deficiency 
alone range as high as 5 percent in this country, the above estimate seems fairly 
conservative. About half of these children, or 2 percent of the total live births, 
are suffering from disorders that have a simple genetic origin and will appear 
prior to sexual maturity. Thus, of the next 100 million children born in the 
United States, two million can be expected to have some sort of hereditary 
defect. These defects are the result of deleterious "spontaneous" mutants induced 
in the past by natural causes and now present in the gene pool of our population. 

Many of these inherited conditions are severe enough to cause the death 
of the child or else to limit or prevent his reproduction. These defective genes, 
then, are constantly being eliminated from the population by natural selection. 
Why, if these genes have been selected against for centuries, are they still so fre- 
quent? The answer is that they are being generated by recurrent spontaneous 
mutations. An equilibrium between their rate of origin by mutation and their 
rate of elimination by selection has been approximated. 

In this connection it may be pointed out that the practice of medicine 
has changed radically in the past 100 years. One hundred years ago the major 
killers of human beings were infectious diseases. Today, where modern medicine 
is practiced, the physician is turning his attention away from combating micro- 
organisms. (The microorganisms have by no means surrendered; the origin of 
resistant strains has tempered the initial optimism that greeted the various anti- 
biotics and chemotherapeutic agents.) The major causes of death at the present 



RADIATION, GENETICS, AND MAN • 353 

time do not involve infectious organisms, but they do involve, to varying degrees, 
harmful genes, the new objects of medical assault. To the extent that the physi- 
cian succeeds in combating the effects of deleterious genes by suitable environ- 
mental manipulations (for example, insulin for diabetes), the results are 
dysgenic, for the proportion of these genes in the population will increase in subse- 
quent generations. The physicians of the next generation, therefore, will have a 
greater proportion of such cases to treat. It is estimated that the average person 
carries the equivalent of about 4 genes, any one of which, in the homozygous 
condition, would cause his death. In other words, he may carry 4 lethals, or 
8 deleterious genes with a 50 percent probability of causing death, or 100 genes 
with only a 4 percent chance. Actually, there is undoubtedly a mixture of these 
types descended from past mutations that still persist in the population. Unless 
some way is found to prevent their increase in frequency, this load of hidden 
mutations will inevitably become heavier as the medical control of genetic defects 
improves. 

The average spontaneous mutation rate for a given gene locus has been 
estimated to be from 1 to 2 new mutations per 100,000 genes per generation. 
This statement means that in 100,000 sperm cells, 1 or 2 can be expected to 
carry a newly arisen spontaneous mutation for a particular gene. However, the 
total rate, a measure of the mutations at all gene loci on all chromosomes, is con- 
siderably higher. The total number of genes is not known in any species. Indirect 
estimates lead to a value of at least 10,000 gene loci in Drosophila, and this 
figure is probably a conservative estimate for man. The total mutation rate there- 
fore equals ■ X 10,000 = . Hence, 1 in 10 gametes or about 2 in 

^ 100,000 10 6 

10 diploid individuals can be expected to carry a newly arisen mutation. At 
equilibrium, this frequency represents the risk of genetic death since the rate of 
elimination of the mutants equals their rate of origin. This risk is shared by all 
of us since everyone carries several to many detrimental mutations. It indicates 
the loss of fitness of the average individual as compared to a hypothetical person 
with no detrimental mutants at all. It should be pointed out that the above esti- 
mates are based primarily on data from Drosophila and mice, with the mutation 
rates in mice somewhat higher than those of the fruit flies. Man's mutation rate, 
because of his greater generation length, is apt to be higher than that of mice. 
More and better data for man are needed, but the fundamental conclusions are 
unlikely to change. 



Genetic Effects of Radiation 

The original discovery of the mutagenic effects of x-rays by Muller in 
1927 was of great interest to geneticists, but only with the coming of the atomic 
age have the biological effects of ionizing radiations become of general concern. 



354 • EVOLUTION AND MAN 

The biological effects are of two major kinds: somatic or direct effects on ex- 
posed individuals, causing either death or immediate or delayed pathological 
effects; and genetic effects in the germ cells of exposed individuals, which are 
then transmitted to subsequent generations. Ionizing radiations such as x-rays 
and gamma rays (similar to x-rays but emanating from radioactive substances 
such as radium) have been shown to induce chromosome breakage as well as 
gene or point mutations. In their passage, these radiations break existing chem- 
ical bonds and lead to the formation of positively and negatively charged ions. 
Presumably the biological effects are the result of the subsequent reactions in 
which these ions are involved. 

Induced mutations are in general similar to spontaneous mutations, 
though chromosome breakage is relatively more frequent among the induced 
mutations. The vast majority of induced mutations are recessive and deleterious 
under existing conditions. There is no threshold dose of radiation below which 
no mutations are induced. Any increase in ionizing radiation above the back- 
ground can therefore be expected to cause a corresponding increase in the num- 
ber of mutations. The mutation rate has been shown to be directly proportional 
to the dosage of radiation. A doubling of the dosage will result in a doubling in 
the number of induced mutations. For chromosomal rearrangements such as in- 
versions or reciprocal translocations, however, the number of rearrangements 



10- 




Fig. 34-2. Relation between x-ray dosage and the frequency of one-hit and 
two-hit chromatid rearrangements. One-hit rearrangements increase in direct 
proportion to the dosage but two-hit rearrangements tend to increase as the 
square of the dosage at relatively high intensities. (With permission of Sax.) 



RADIATION, GENETICS, AND MAN • 355 

increases more nearly as the square of the dosage. This difference is attributed 
to the fact that two independent breaks are required for the rearrangements, 
whereas mutations are single-hit events. See Fig. 34-2. 

Although an intensity effect has recently been reported in mice, the 
number of gene mutations induced is usually independent of the intensity 
with which the radiation is delivered. One hundred roentgens in 5 minutes 
causes the same number of point mutations as lOOr in 5 months or 5 years. A 
roentgen of x- or gamma radiation is the amount that will, when applied to air 
at standard conditions (0° C, 760 mm mercury), produce 2.1 X 10 9 ion pairs 
per cubic centimeter (1 electrostatic unit of charge). In water or tissue, the 
number of ion pairs produced by If is estimated to be about 800 times greater. 
Much easier to remember is the fact that It causes approximately 2 ionizations 
per cubic micron of tissue. The effect of the radiation, then, is cumulative, for a 
mutation, once it occurs, does not heal, but is self-duplicating and persists until 
it causes a genetic death. 



Somatic Effects of Radiation 

A severe exposure to radiation may be lethal. The acute lethal dose for 
50 percent of the exposed individuals (the so-called L.D. 50) has been estimated 
in man to be in the range of from 400 to 600r. Lesser doses produce a variety of 
somatic effects, and the parts of the body where cell division is rapidly occurring 
appear to be particularly sensitive. Early symptoms, for example, among the sur- 
vivors of the explosions at Hiroshima and Nagasaki were disturbances of the 
gastrointestinal tract and in the blood-forming tissues. Temporary or in some 
instances permanent sterility may be induced. Later effects of acute exposure or 
of low-level chronic exposure include skin cancer and leukemia, which may not 
develop until long after the exposure. Finally, in addition to these rather specific 
ailments, there are nonspecific effects such as a lower immunity to disease, 
damage to the connective tissue, and signs of premature aging. In mice, the 
most sensitive index of somatic damage is the shortening of the life span. 

All of the biological effects of radiation mentioned thus far have been 
observed in man except for one, the induction of gene mutations. Skin cancer, 
leukemia, etc., are known consequences of exposure to radiation that have been 
made more or less familiar by newspaper reports. Shortening of the life span is 
indicated by data on radiologists (Table 34-1). Even the breakage of human 
chromosomes has been demonstrated in human cells in tissue culture by Bender. 
In view of these effects in man, there is no reason to suppose that he has some 
sort of mysterious immunity to the mutagenic effects of radiation. Why, then, 
has it not yet been demonstrated ? 



356 • EVOLUTION AND MAN 



TABLE 34-1 
Effects of Radiation on Life Span 



Group Average age at death 

U.S. population over 25 65.6 

Physicians (not exposed) 65-7 

Physicians (some exposure — urologists, dermatologists, etc.) 63.3 

Radiologists 60.5 

* National Academy of Sciences, 1956. "The biological effects of atomic radiation," Summary 
reports. Washington, D. C. 



Radiation Effects in Man 

The largest study of the genetic effects of radiation was made under 
the auspices of the United States Atomic Energy Commission on the children of 
the survivors of the explosions at Hiroshima and Nagasaki. Among these chil- 
dren, as compared to the controls, there were no statistically significant increases 
in the number of stillbirths or abnormalities but a possible slight effect on the 
sex ratio was reported. The Genetics Conference that set up the project expected 
these results from the outset, but the opportunity for such a study was unique 
and it seemed wise to seize it. 

Let us consider the reasons for the lack of significant differences be- 
tween exposed and control populations. The unexposed controls showed more 
than 1 percent visible, though slight, malformations at birth, a part of the 4 to 
5 percent defective mentioned previously. Furthermore, less than 1 in 100 in- 
duced mutants are dominant and will be immediately expressed in the next 
generation. Therefore, it has been estimated that out of 1000 children whose 
parents both received lOOr (that is, the most heavily irradiated survivors), it 
can be expected that 30 percent will carry a newly induced mutation. However, 
only about 1 percent of these mutations will be dominant and expressed in the 
children. Simple arithmetic (1000 X 0.3 X 0.01 = 3) shows that only 3 among 
the 1000 childen can be expected to be malformed at birth due to the irradiation. 
Since 1 percent or 10 in 1000 can be expected to be malformed at birth due to 
causes other than the radiation, a statistical comparison is required between 10 in 
1000 and 13 in 1000. Obviously, such a small difference will be subject to ran- 
dom fluctuations unless very large numbers are available for study. Special 
genetic techniques that are, fortunately, not available to the human geneticist 
would be needed to reveal the much more numerous induced recessives. How- 
ever, lack of the techniques is no reason to suppose that mutations have not oc- 
curred and been added to the existing load of mutations. Other studies of the 
children of radiologists and of children whose parents have received therapeutic 
pelvic irradiation of lOOOr or more (skin dose) have indicated a genetic effect. 



RADIATION, GENETICS, AND MAN • 357 

These data were assembled by questionnaire and are possibly subject to bias since 
the returns were not complete. One conclusion that can be drawn, perhaps, from 
the genetic studies is that man cannot be much more susceptible to radiation than 
are mice. 

A useful way to look at the problem is in terms of the doubling dose, 
that amount of radiation which will induce as many mutations as now occur 
spontaneously. The doubling dose was independently estimated by two groups 
in the United States and Great Britain with surprisingly good agreement as 50r 
and 30 to 80r. In other words, if the population of the United States were sub- 
jected to an additional 50r per generation, the number of children born with 
genetic defects would gradually rise from 2 percent to 4 percent as the new 
equilibrium is reached. Taking all factors into account, the National Academy of 
Sciences has recommended that the total accumulated dose of ionizing radiation 
from humanly controllable sources to the reproductive cells from conception to 
age 30 should not be more than lOr. This recommended dose is by no means 
harmless but is considered reasonable. However, for 100 million children an 
increase of lOr is estimated to give rise to 50,000 new inherited defects in the 
first generation and ultimately at the new equilibrium to 500,000 per generation. 
Clearly, any increase at all must be regarded as harmful. Recent estimates for the 
average exposure to radiation of the gonads of the population of the United 
States are as follows: 

Source of radiation 

background 3.1r per 30 years 

medical uses of radiation 4.6r per 30 years 

fallout from atomic explosions O.lr per 30 years 

There apparently is a threshold for most somatic effects of radiation, 
for with two possible exceptions, doses several times as large as the recom- 
mended lOr limit are necessary to cause detectable somatic damage. One possible 
exception is the shortening of the life span. Even though doses of up to lOOr 
spread over a period of years have not been shown to shorten human life, it is 
still possible that there is no threshold. If, for example, large numbers of people 
exposed to a gradually accumulated dose had their life expectancy lowered very 
slightly, the individual effect might seem trivial, but the total effect would be 
very great. 

The other possible exception is the effect of strontium-90. This radio- 
active element, rather similar chemically to calcium, tends to accumulate in bone. 
The major hazard from Sr 90 is the internal radiation of the red bone marrow, 
which may lead to the development of leukemia. The maximum permissible con- 
centration (MPC) of Sr 90 in man has been set at 1 microcurie per 1000 grams 
of calcium. (A microcurie produces an amount of radiation equivalent to that 
emanating from a millionth of a gram of radium. The body of the average 



-c 



358 • EVOLUTION AND MAN 

human adult contains about 1000 grams of calcium.) Just 0.1 of the MPC would 
give a dose rate of 0.1 to 0.2r per year to the red bone marrow. For the present 
population of the United States, the expected number of additional cases of 
leukemia at this dosage level would be 500 to 1000 per year. Since there are 
currently about 10,500 deaths from leukemia in the United States each year, one- 
tenth the MPC of Sr 90 would be expected to increase the present incidence of 
leukemia about 5 or 10 percent. However, the present levels of Sr 90 in bone are 
about 1/1000 rather than 1/10 of the MPC, and therefore Sr 90 cannot now be 
regarded as a major hazard to the human population; the level of Sr 90 in bone 
must be watched, however, for if it rises, the hazard will increase. Furthermore, 
it seems unlikely that the existing levels of exposure are causing any major 
shortening in the human life span. However, there is no question that much 
additional research is needed to back up the available estimates and to clarify 
still further the somatic effects of radiation. 

The major hazard at the present time is the genetic effect of radiation, 
and the major source of man-made radiation for the population of the United 
States is the medical use of radiation. The amount received currently from fall- 
out is only 1 or 2 percent as great as the amount received in the course of the 
various medical uses of ionizing radiation. While some scientists have greatly 
emphasized the dangers inherent in nuclear weapons testing, others equally 
reputable have suggested that the dangers are trivial or nonexistent or may even 
be beneficial. Under these circumstances the public cannot be blamed for being 
somewhat confused about the hazards involved. A true concern for human wel- 
fare would seem to dictate that the problem of radiation hazard must be faced 
as a whole, and that the solution must encompass not only nuclear tests but the 
medical and industrial uses of radiation as well. The evidence now available 
indicates quite clearly that the net effect of any increase in the exposure of the 
human population to radiation will be harmful. However, it is also clear that 
more research is desirable and necessary to delineate more specifically just how 
great are the hazards to man. 

The varied uses of radiation raise questions to which there are no simple 
answers. For the physician, each use of radiation requires that he weigh the im- 
mediate benefits to his patient against the possible genetic damage to future gen- 
erations. And this, of course, raises the question of just what are our obligations 
to future generations. Is it possible that the doctors of another day will be able 
to mend damaged genes as they now mend broken legs? If it is possible, how 
much radiation can the human species safely absorb until that day comes? The 
weapons tests similarly require an evaluation of the benefits and hazards of test- 
ing versus not testing. Unfortunately, the decisions on testing are based in the 
final analysis on political rather than on scientific or humanitarian considerations. 



RADIATION, GENETICS, AND MAN • 359 

SUGGESTED READING 

Effect of radiation on human heredity, 1957. Geneva: World Health Organization. 

"Ionizing radiation," Scientific American, 201(5) Sept. 1959. 

Medical Research Council, 1956. "The hazards to man of nuclear and allied radia- 
tions," Cmd. 9780. London: H. M. Stationery Office. 2d Report, I960. 
Cmnd. 1225. 

Muller, H. J., 1950. "Radiation damage to the genetic material," Amer. Scientist, 
38:33-59; 399-425. 

, 1950. "Our load of mutations," Amer. Jour. Human Genetics, 2:111-176. 

National Academy of Sciences, 1956. "The biological effects of atomic radiation," 
summary reports. Washington, D.C. 2d Report, I960. 

Wallace, B. and Th. Dobzhansky. 1959. Radiation, genes, and man. New York: 
Holt, Rinehart and Winston. 



CHAPTER 



35 



Man as a Dominant Species 



The human population is subject to the effects of natural 
selection, mutation, gene flow, and random genetic drift just as 
are the populations of other species. In the future as in the past, 
the qualitative characteristics of the human population during the 
course of its evolution will be determined by the net effect of the 
action of these factors. However, in addition to changing qualita- 
tively, the human population may also change quantitatively. The 
most noteworthy aspect of human biology in the last few centuries 
has been the tremendous increase in the size of the human popu- 
lation, an increase of such overriding significance that any con- 
sideration of human affairs that fails to include it is seriously 
deficient. 

The population problem is an involved, controversial, 
and paradoxical subject, so beset by emotion and prejudice that 
discussing it objectively is far more difficult than discussing fac- 
tors that regulate the numbers of grasshoppers or deer or field 
mice. There are two schools of thought about the hazards of 
man's increasing numbers. One group will state flatly that Malthus 
has long since been proven wrong, that man can produce all the 
food and goods necessary for any possible increase in his numbers, 
and that his ingenuity and resourcefulness (or science and tech- 
nology) will insure that production will more than keep pace 
with population growth. Any present difficulties in getting suffi- 
cient food and other necessities are attributed to a failure in the 
system of distribution rather than to overpopulation. One cannot 
help but wonder at times whether these people have ever read the 

360 



MAN AS A DOMINANT SPECIES • 361 

words of Malthus whom they so readily dismiss. Opposed to this group 
is another group, who will point out that right now three-fifths of the 
world's people are living at a bare subsistence level, and that since we 
can not even take care of our present population in a satisfactory way, there 
is no reason to suppose that we can do so in the future if the present rate of 
increase continues. The question is whether the earth's resources are sufficient to 
support the present population and the potential future population at a standard 
of living above the bare subsistence level. The future of mankind may well hinge 
more on the answer to this question than on any other single factor. 

In order to make an objective appraisal of the pros and cons of this 
question, certain relevant facts must be reviewed. All living organisms, including 
man, are ultimately dependent for their very existence on the photosynthetic 
processes of green plants by which the sun's energy is utilized to form organic 
materials (that is, food) from simple inorganic compounds. This fact is inescap- 
able at the present time, and it appears unlikely that other means of synthesizing 
food in significant quantities will be devised in the near future. The maximum 
size of the human population, then, ultimately depends on the amount of food 
that can be grown to support it. The areas available on the earth in which food 
might be grown consist of the following: 

A. Land 1. Fertile regions 33,000,000 square miles 

2. Steppes 19,000,000 square miles 

3. Deserts 5,000,000 square miles 

B. Water 140,000,000 square miles 

This is all there is; there isn't any more. (The implications of the space age can 
safely be ignored in the present discussion, for the problems of transportation 
and distribution have not yet been successfully solved here on earth and will be 
infinitely greater in any interplanetary situation.) Crops can only be raised in the 
fertile regions. The vegetation of the steppes is made available to man through 
its use as pasture; the vegetation of the seas is the pasture, in a sense, of the 
fishes. The amount of fertile land can be increased through irrigation. The yield 
can be improved through improved agricultural methods and the use of im- 
proved varieties of plants and animals. These changes have been and are con- 
tinuing to be made in many parts of the earth with spectacular success in many 
instances in increasing the productivity of the land. 

Man has existed for at least several hundred thousand years. Although 
exact figures are not available, the best estimates indicate that until 1650 human 
population growth was relatively slow and erratic. By that time the human popu- 
lation was estimated to be about 500 million. In less than 200 years, by 1825, 
world population had doubled, and for the first time more than a billion people 



362 • EVOLUTION AND MAN 

inhabited the earth. In another 100 years the population had again doubled to 
2 billion. In the few decades since 1925 this growth has continued, until the 
present world 'population is estimated to be over 2.8 billion people. Thus from a 
species of limited range and numbers, man has seemed almost literally to explode 
over the face of the earth. He is now a cosmopolitan species, yet it seems likely 
that 50,000 years ago North and South America were completely uninhabited by 
man, and that in the inhabited areas the population density was low, typical of a 
hunting or nomad population. See Fig. 35-1. 



Millions 



l ining 



World population growth 
5000 BC-1950 AD 



I l I I M I I I 



3500 

--3000 
-- 2500 

--2000 

1500 

1000 

--500 
■• 100 



Year 



4000 



3000 



2000 



1000 BC 



1 AD 



1000 



2000 



Fig. 35-1. World population growth, 5000 B.C. to 1950 a.d. Not only the size 

of the human population but the annual rate of growth has increased markedly 

since 1800. (With permission of Sax.) 



Not only has the human population increased, but it has increased at an 
accelerating rate. The annual rate of increase has grown from an estimated 
0.4 percent between 1650 and 1850 to 0.8 percent between 1850 and 1950, and 
is currently estimated to be about 1.7 percent per year. The numerical increase is 
thought to be nearly 45 million a year or about 123,000 per day. Projection of 
these figures into the future has led to estimates of 6 billion people by the year 
2000 and nearly 13 billion by 2050. The facts are, then, that we have on the 
earth a limited amount of space and fertile land on which to support a human 
population rapidly growing at an accelerating pace. Obviously this growth cannot 
and will not continue indefinitely and these figures may never be reached. How- 



MAN AS A DOMINANT SPECIES • 363 

ever, the way in which this trend is slowed or reversed will have a tremendous 
impact on the future welfare and happiness of mankind. 

Elementary Demography 

Under favorable conditions, the human population could easily double 
every 25 years. The fact that it has not done so is an indication that man's exist- 
ence has been rather precarious, with disease, pestilence, famine, natural catastro- 
phes, and war all having exacted a heavy toll in the past. The size of any popu- 
lation is determined by the relationship between the death rate and the birth 
rate, and even though birth rates were high in the past, death rates were also 
high so that growth of the human population was slow and irregular. The most 
common way to express birth rates or death rates is in terms of the number of 
births or deaths per 1000 population, the so-called crude birth and death rates. 
Since both birth rates and death rates vary with age, the crude rates will also 
depend on the age structure of the population and may not be directly com- 
parable in two populations having different age distributions. 

The rapid population growth in the Western world during the last few 
centuries has been due to the scientific revolutions in the fields of public health, 
agriculture, and industry. The initial effect of these revolutions was a reduction 
in the death rate, and this can be attributed primarily to the revolution in medi- 
cine and public health. Many diseases have been eliminated or brought under 
control so that infant mortality has been reduced from about 200 to about 30 per 
thousand infants and the average crude death rate has fallen from about 40 to 
about 12 or less per thousand. As a consequence, life expectancy at birth has 
risen from between 25 and 30 to between 60 and 70 years. 

The revolution in agriculture has resulted from mechanization and from 
scientific advances in plant and animal breeding as well as in the methods 
of cultivation and fertilization; yield per acre and also yield per agricultural 
worker have risen dramatically among the Western nations. In the United States 
in 1700, for example, it took 4 farm families to produce enough food for 5 fam- 
ilies. Today one farm family produces enough food for 6 families, or for 10 
families living at the standards of 1700. Therefore, since the efforts of 5 out of 
6 families can now be diverted from the production of food into the production 
of other goods and services, the standard of living has risen rapidly. 

The industrial revolution, which went more or less hand in hand with 
the agricultural revolution, increased the food supply through the mechanization 
of farming and through the improved transportation system, by which food 
could be shipped from areas of high production to areas of consumption where 
it was exchanged for manufactured products. Emigration from crowded regions 
in Europe to empty lands in America and elsewhere overseas became possible, 
and helped to relieve the pressure of an expanding population. 



364 • EVOLUTION AND MAN 



Still another transition has taken place in most of the Western nations, 
perhaps as revolutionary as any thus far mentioned. This revolution, more recent 
in onset than the others, has resulted in declining birth rates. As a result of the 
time lag between the fall in the death rates and the fall in the birth rates, the 
so-called demographic transition from a high birth rate-high death rate agri- 
cultural society to a low birth rate-low death rate industrial society has always 
historically been accompanied by a rapid increase in population size (see Fig. 
35-2). When the death rate is lower than the birth rate, the difference between 
the two can be regarded as a measure of the net increase; when the birth rate 
falls below the death rate, the population will, if this relation persists, decline in 
numbers. 



STAGE I 

High Fluctuating 

(HIGH BIRTH AND ', 
DEATH RATES) ^ 



STAGE II 

Early Expanding 

(HIGH BIRTH RATE5,DECLINING DEATH RATES) 



STAGE III ! STAGE IV 

Late Expanding ! Low Fluctuating 

(DECLINING BIRTH !(LOW BIRTH AND 
AND DEATH RATES); DEATH RATES) 




Rate per 
thousand 



Dates 



Population 6*5 

(miuions) 



1800 
9 



1850 


1880 


1900 


1930 


1950 


18 


26 


32-5 


40 


43-5 



Fig. 35-2. The demographic transition in England and Wales from a society 

with high birth and death rates to one with low rates of births and deaths. 

(With permission of P.E.P. Report. World Population and Resources.) 

During the demographic transition, various stages can be recognized. 
Initially there is an agricultural society, with high birth rates, high death rates, a 
slow and irregular increase in numbers, and a relatively low standard of living. 
In the next stage the death rate starts to fall quite rapidly while the birth rate 
continues high. The decline in the death rate comes first because the measures 
needed to control the death rate are relatively simple and easy to put into effect. 
Thus the initial impact of modern scientific knowledge on a backward society 
has been on the death rate. The sensitivity of the death rate to changed condi- 



MAN AS A DOMINANT SPECIES • 365 

tions can be illustrated by the spectacular drop in Japan from a death rate of 
32 per 1000 in 1945 to 12 per 1000 in 1948. In Ceylon an antimalarial cam- 
paign using DDT brought the death rate from 20.2 in 1946 down to 14.2 in 
1947 and to 9-8 by 1956. In 1946 there were 12,578 deaths from malaria; in 
1947, 4557; and in 1956, 144. Life expectancy at birth rose from 45.8 years to 
over 60 years. Obviously when death rates decline in this fashion and a decline 
in birth rates does not immediately follow, the population increase is rapid. 

During the next phase, the birth rate also starts to decline rather rapidly 
while the death rate continues to fall. The causes of declining birth rates have 
never been clearly defined, but, and here is the paradox, birth rates have started 
to fall in the past only after standards of living have improved. Thus, birth rates 
are highest in just those areas where people are least able to support large fam- 
ilies. During this period when both birth and death rates are declining, popula- 
tion continues to increase but at a decelerating pace. 

The final stage, reached when the demographic transition is completed, 
is marked by low birth and death rates and near equilibrium conditions. Usually 
the birth rate remains somewhat higher than the death rate so that the population 
continues to grow at a slow rate. In countries that have made the transition, 
standards of living are high, life expectancy is long, and birth rates are very 
sensitive to economic forces. 

Burma may be cited as an example of an underdeveloped nation with 
high birth rates (47.5, 1951-53), high death rates (35.7, 1951-53), and a rather 
slow rate of growth (though declining death rates may lead to more rapid 
growth). The island of Mauritius appears to have reached the second stage, since 
the birth rate (1949-53) was 46.5 while the death rate was only 14.9, the result 
being a sizable natural increase. Puerto Rico has recently reached the third phase 
of the demographic transition, for her birth rate had fallen (1953) from be- 
tween 40 and 50 to 34.8 and the death rate to 8.1. The United Kingdom has 
essentially completed the change, for in 1953 the birth rate was 15.9 and the 
death rate was 11.4, and the rate of natural increase was quite low. 

In Western Europe the change from a high birth rate-high death rate 
society to a low birth rate-low death rate society brought about a sixfold increase 
in population. North America had a sixfold increase in just a single century, 
between 1850 and 1950. Japan has made the transition in less than a century, 
more rapidly than any other nation, and yet, despite the speed of the change, 
has almost tripled from about 35 million in 1868 to nearly 100 million today. 

In spite of their increases in population size, the nations of the Western 
world have had a notable rise in their standards of living. The economic well- 
being of the people of these nations is higher than it has ever been anywhere. 
This fact, that standards of living have increased while populations were grow- 
ing rapidly, has led, it seems clear, to the optimistic view that Malthus was 
wrong. However, his basic statements were: 



f 



366 • EVOLUTION AND MAN 

1. Population is necessarily limited by the means of subsistence. 2. Popula- 
tion invariably increases, where the means of subsistence increase, unless prevented 
by some very powerful and obvious checks. 3. These checks, and the checks which 
repress the superior power of population, and keep its effects on a level with the 
means of subsistence are all resolvable into moral restraint, vice and misery. 

He distinguished between preventive checks, which tended to reduce the 
birth rate, and positive checks, which raised the death rate. The essential sound- 
ness of his position seems clear. What he did not foresee was the possibility that 
preventive checks could come to be as significant as they are in some nations 
today. 



The Causes of Overpopulation 

Because of the revolutions in agriculture and industry, the means of sub- 
sistence in the Western world have increased even more rapidly than has the 
population, and the West has managed thus far to escape the Malthusian devil 
of overpopulation. The meaning intended here for the term "overpopulation" 
is that there are more people than can be supported at a reasonable standard of 
living on the available resources (of all kinds) in the area. The implication is 
that if the population had not grown so large, the people individually would be 
better off, and if it continues to grow, living standards will fall still further. It 
will be worthwhile to examine the routes by which the West has escaped this 
situation and to evaluate their applicability to those areas of the world that have 
yet to make the transition. 

When such an analysis is made, it becomes obvious that the three-fifths 
of the world's people who have a low living standard (per capita income usually 
less than $100 per year), an average length of life in the 30's, a high birth rate, 
and a low literacy rate cannot hope to escape from overpopulation by following 
the same sequence of events as the Western world. This statement may seem 
rather dogmatic and therefore warrants further more detailed consideration and 
discussion. 

The first impact of modern scientific knowledge on a backward agricul- 
tural society has always been on the death rate, because public health measures 
such as sewage disposal, water purification, mosquito control, vaccination, etc., 
are relatively inexpensive and easy to institute. However, the longer life will not 
necessarily be a happier one, for countries such as India, China, and Egypt are 
already densely populated and cannot hope to support even a twofold increase in 
population at a higher standard of living, let alone a threefold, or sixfold, or 
tenfold increase. Efforts toward industrialization are beset by the fact that there 
are few areas (the United States is one) with excess food to exchange for manu- 
factured products, and these areas may not need or want the manufactured goods. 
Furthermore, markets are not as readily available as they were 150 years ago. If 



MAN AS A DOMINANT SPECIES • 367 

it is argued that the primary need is to increase agricultural production rather 
than industrial expansion, another dilemma presents itself. This situation can 
best be clarified by an actual example. It might be expected that a marked rapid 
increase in the food supply would give the farmers a surplus that could then be 
exchanged for manufactured goods and for services so that their living standards 
would rise. This argument, in one form or another, seems to be the one that has 
led some individuals to view the population problem with equanimity. However, 
it ignores the demographic effects of an increased food supply in an under- 
developed country, and therein lies its fallacy. On the Malabar coast of India, 
rice had been the staple food crop for centuries until research showed that tapioca 
(cassava) was a more profitable crop for this area. The change to tapioca was 
put into effect rapidly, and food production was approximately doubled in a few 
years. In just 12 years, however, the population in this area had also doubled, so 
that twice as many people now lived on twice as much food at the same bare 
subsistence level. Therefore, even though various governments have set up five- 
year plans or other programs designed to increase agricultural production or to 
encourage the development of industry, such programs may not resolve the prob- 
lems, even when their goals are achieved, if the demographic factors are not 
favorable or are ignored in the planning. In fact, the situation may actually be- 
come worse than before. 

The pressure of the growing population in Europe during the demo- 
graphic transition was relieved in part by the emigration of large numbers of 
Europeans to America and to other parts of the world that were then sparsely 
populated. The safety valve provided by emigration is no longer available, for 
there are no more large unoccupied habitable areas in the world. Furthermore, 
the very magnitude of the logistic problems involved makes it clear that the 
solution for overpopulated areas is not to export their surplus population (even 
if they were able to decide who was surplus and who was not). In India, for 
example, a series of favorable crop years between 1931 and 1941 led to an 
increase of 50 million in her population, an average of 5 million per year. 
Imagine, if you will, the problems involved merely in transporting 5 million 
people per year from India to some other part of the world, not to mention the 
problems of finding housing and jobs for them in their new environment. The 
United States, at the peak of its all-out effort in World War II, transported and 
supported overseas only about 8 million men. Clearly, the relocation of millions 
of people, the numbers about which we must think, would be impossible, espe- 
cially for those nations whose resources are already strained by overpopulation. 
Hence, this solution holds little promise for the present problems even if un- 
developed lands were available. A further complication should also be pointed 
out to indicate another aspect of the problems created by migration. Existence of 
the bitter racial tensions that have developed between white and Negro in South 
Africa is probably familiar to readers. Less well known, perhaps, is the fact that 



368 • EVOLUTION AND MAN 

South Africa also has a fairly large and rapidly growing population of immi- 
grants from India. This emigration has had no noticeable effect on the rate of 
growth of the Indian population. However, the migrants took their low living 
standards and high birth rates with them to South Africa, thereby arousing the 
resentment of both black and white, and the troubles of South Africa are now 
being compounded by a three-way racial tension. In a sense the problem has been 
transplanted rather than solved. 

The Regulation of Man's Increasing Numbers 

The final possible solution to the problems of the three-fifths of the 
people who live in either the first or the second stages of the demographic transi- 
tion is to reduce the birth rate in step with the reduction in the death rate so 
that numbers remain stabilized. This solution seems to be the one with the great- 
est chance of success, yet it is by far the most difficult to put into effect. 

The situation in these areas is distinctly different from that in Europe 
two centuries ago. Death rates not only can be but have been brought down 
drastically in a very short period by the application of modern scientific knowl- 
edge in backward areas, and the decrease has been much more rapid than it ever 
was in Western Europe. Consequently, the potential explosive increase in popu- 
lation size that exists in these areas is far beyond what ever occurred in Europe. 
It is also possible to increase production in agriculture and industry in these 
areas, although more time and effort are required than is needed to reduce the 
death rate. However, a reduction in the birth rate takes much longer and is much 
more difficult to achieve than is the control of deaths or an increase in produc- 
tivity. In the past, birth rates have started to fall only after standards of living 
have been raised. The highest birth rates are associated throughout the world 
with high levels of poverty and ignorance. If the historical sequence of events 
is followed in the underdeveloped countries today, the outcome would appear to 
be different from that in the Western nations. The reproductive potential is so 
great that population increases, before they can raise living standards to the point 
where birth rates might be expected to decline, will absorb any increase in pro- 
duction. As a consequence, more and more people will be supported at a bare 
subsistence level. The contrast between the nations that have made the demo- 
graphic transition and those that have not will become even more stark, and the 
explosive possibilities of such a situation on the international scene can hardly 
be minimized. The conclusion seems inescapable, therefore, that countries today 
that have high birth and death rates and that wish to better the lot of their 
people and their positions as nations must direct their efforts toward bringing 
birth rates under control. 

Only if the population growth can be held down can increased produc- 
tion be used to improve living conditions. In the absence of checks on growth, 



MAN AS A DOMINANT SPECIES • 369 

natural increase rather than living standards responds to economic development. 
The mere development of underdeveloped countries has never been shown 
capable in itself of raising living standards. Self-generated development is usu- 
ally slow because of the difficulty in amassing sufficient capital and resources to 
speed the process. As a result, the population increase rapidly absorbs the gains 
as they are made, and as the population grows, the problem of making the demo- 
graphic transition becomes increasingly difficult. External aid on a massive scale 
has been suggested as a possible solution. However, outside aid, whether in the 
form of capital, equipment, or technical aid or training, is equally unlikely to be 
effective if unaccompanied by some means of limiting the increase in population. 
The experience of the British in India and Egypt and of the United States in 
Puerto Rico point up some of the problems involved. In the decades of rule by 
the British in both India and Egypt, during which the gross national product of 
the countries undoubtedly increased, population growth more than kept pace 
so that today living standards in these countries are probably lower than they 
were 50 or more years ago. The United States has poured over a billion dollars 
in aid into Puerto Rico since assuming control in 1898 — the greatest effort ever 
made to put a backward nation on its feet through outside assistance. The most 
obvious result of this aid has been an increase in population from about a million 
to more than two and a quarter million. The death rate per 1000 declined gradu- 
ally from 31.4 in 1899 to below 10 per 1000 in recent years while the birth rate, 
which was over 40 per 1000 in 1899, remained high until about 1947 when a 
slow decline set in. The actual natural increase is still about 60,000 per year. 
Emigration to the United States has served as a safety valve, for in recent years 
annual net emigration has almost equaled the natural increase, thus stabilizing 
the population size. Some progress toward raising the standard of living has 
been made since about 1945. However, unemployment is still common, and 
housing and schools are still inadequate. Thus after 60 years of generous aid, 
limited results are finally forthcoming, but Puerto Rico has occupied such a 
uniquely favorable position that the picture can hardly be considered encouraging 
with respect to what might be done for other less well-situated areas. Only after 
40 years did signs of progress appear, and Puerto Rico's problems are by no 
means solved yet. What then can the prospects be for the much larger under- 
developed nations that can find no place to export their surplus population and 
cannot hope to receive outside aid on the same scale as was used in Puerto Rico ? 
The answer clearly is that the primary task in the development of the 
have-not nations of the world is the reduction of the birth rate along with the 
death rate so that population explosions are not detonated across the surface of 
the earth. Reduction in the birth rate must accompany the agricultural, industrial, 
and medical revolutions, and not lag behind. The pattern of the past will some- 
how have to be broken. To do so will not be easy, for it represents a major effort 
in educating peoples who are illiterate, poverty stricken, and hunger ridden, and 



370 • EVOLUTION AND MAN 

usually not particularly interested in this type of education. The task may be 
further hampered by religious, ethical, or moral scruples and by legal or political 
barriers to the dissemination of such information. It may involve educating not 
only the common people but their leaders, for before the solution can be at- 
tempted, the problem itself must be clearly recognized and generally understood. 
Since fertility has customarily been admired in most societies in the past, a major 
shift in attitude will be required of many peoples. The freedom to have children 
must certainly be ranked with the Four Freedoms or any other of the basic 
human rights. In fact, it might well be argued that the right to reproduce is the 
most fundamental of all human rights. Therefore, any program designed to re- 
duce the birth rate must, if it is to be in accord with democratic principles, some- 
how be based on the voluntary cooperation of each couple rather than enforced 
by decree. 

The Roman Catholic Church is often pictured as being opposed to con- 
trol of the birth rate; this is, in fact, not so, for the Church approves of such 
control in principle but is opposed to certain of the methods, whch are consid- 
ered "unnatural." It is to be hoped that other religions and other cultures will 
also approve in principle and that effective methods for control will be found 
that are acceptable to the great majority of the peoples of the world. Much 
research still needs to be done in this area, but present results indicate that 
simple, inexpensive, and effective methods may soon be available. 

Lest those nations not now troubled by overpopulation or likely to be in 
the foreseeable future stand aside and regard the problem as not being a matter 
of concern to them, the genesis of World War II should be recalled. In essence, 
three nations, each nearing completion of the demographic transition, attempted 
to relieve their growing population pressure by expansion. Germany sought 
Lebensraum to the east in Poland and the Ukraine, Italy expanded into North 
Africa, while Japan overran China and many of the Pacific islands. The in- 
stability and dissatisfaction generated in overpopulated areas will continue to be 
a threat to world peace, for human dignity, human rights, and human life have 
little value or meaning in these areas. Therefore, overpopulated areas should be 
a matter of concern to all, and steps must be taken to raise living standards 
through agricultural and economic development. However, unless population 
increase is controlled, all such efforts seem destined to failure. The most hopeful 
development in recent years is that the governments of Japan and India, two 
nations beset by the problems of more people than resources with which to 
support them, have officially recognized the problem and have taken steps to aid 
their people in limiting the size of their families. The experience gained in these 
countries and their degree of success will be of great interest and significance to 
the rest of the world in its search for a better and a happier life for all mankind. 

Another solution to the problem of overpopulation is suggested in a 
passage written by Hendrik Willem van Loon nearly thirty years ago. 



MAN AS A DOMINANT SPECIES • 371 




Fig. 35-3. One possible solution to the population problem. (With 
permission from Van Loon's Geography.) 



372 • EVOLUTION AND MAN 

It sounds incredible, but nevertheless it is true. If everybody in this world 
of ours were six feet tall and a foot and a half wide and a foot thick (and that is 
making people a little bigger than they usually are), then the whole of the human 
race (and according to the latest available statistics there are now nearly 
2,000,000,000 descendants of the original Homo sapiens and his wife) could be 
packed into a box measuring half a mile in each direction. That, as I just said, 
sounds incredible, but if you don't believe me, figure it out for yourself and you 
will find it to be correct. 

If we transported that box to the Grand Canyon of Arizona and balanced 
it neatly on the low stone wall that keeps people from breaking their necks when 
stunned by the incredible beauty of that silent witness of the forces of Eternity, and 
then called little Noodle, the dachschund, and told him (the tiny beast is very intel- 
ligent and loves to oblige) to give the unwieldy contraption a slight push with his 
soft brown nose, there would be a moment of crunching and ripping as the wooden 
planks loosened stones and shrubs and trees on their downward path, and then a 
low and even softer bumpity-bumpity-bump and a sudden splash when the outer 
edges struck the banks of the Colorado River. 

Then silence and oblivion. 

The human sardines in their mortuary chest would soon be forgotten. 

The Canyon would go on battling wind and air and sun and rain as it has 
done since it was created. 

The world would continue to run its even course through the uncharted 
heavens. 

The astronomers on distant and nearby planets would have noticed noth- 
ing out of the ordinary. 

A century from now, a little mound, densely covered with vegetable 
matter, would perhaps indicate where humanity lay buried. 

And that would be all. 

Let us hope that it never comes to this. However, if perchance one 
starry-eyed young couple were somehow overlooked and if they then doubled 
their numbers every 25 years for just 32 generations, in 800 years they would 
have over 4 billion living descendants. Such, as Malthus might say, is the power 
of population. 

SUGGESTED READING 

Darwin, C. G., I960. "Can man control his numbers?" Evolution after Darwin, 
Vol. 2, The evolution of man, Sol Tax, ed. Chicago: University of Chicago 
Press. 

Malthus, T. R., 1798. Essay on population, 1st ed. Ann Arbor Paperbacks (1959). 
4th ed., 1807. 

Population bulletin. Washington, D. C: Population Reference Bureau. 

Van Loon, H. W., 1932. Van Loon's Geography. New York: Simon and Schuster. 

World population and resources, 1955. Fairlawn, N. J.: Essential Books. 



CHAPTER 



36 

Man's Future 



Predictions are so often wrong, even about such relatively 
simple matters as horse races or football games, that the effort to 
make them hardly seems worthwhile. However, forecasts continue 
to be made, perhaps for the prognosticator's occasional satisfac- 
tion in being right, more probably as a guide in determining a 
course of action. Since the question of man's future is extremely 
complex, anyone embarking on this sort of crystal-gazing expedi- 
tion should go well equipped with a supply of conditional clauses. 



Man's Future as a Species 

One basis for predicting the future is to examine the 
past. The first conclusion to be drawn from the past is that more 
than 99 percent of all animal species have become extinct. Some 
of them disappeared in the process of evolving into something 
different, but most of them came to a complete dead end; extinc- 
tion was final and irrevocable. Since there is really no reason to 
suppose that man has a tighter grip on immortality than any other 
species, the chances seem quite good that the ultimate fate of 
Homo sapiens, like that of Neanderthal man, will be extinction. 
After all, men like ourselves did not become common on the face 
of the earth until less than 50,000 years ago, a mere drop in the 
bucket of time. 

From quite another point of view, the evolutionary line 
that has given rise to man has persisted for millions and millions 
of years, and it might therefore be expected, on the basis of its 



373 



374 • EVOLUTION AND MAN 

previous success, to persist a while longer. In this event, however, in view 
of the rapid rate of evolution in the Hominidae during the past million 
years, Homo sapiens can be expected to continue to evolve, eventually 
into an hominid population sufficiently different from Homo sapiens to 
be recognized as a new species. In either case, man as we know him today 
seems unlikely to persist indefinitely. This you may regard as fortunate or un- 
fortunate, depending upon your point of view. Although we may prefer to think 
that man in some form will continue to exist, the realization that we are not 
immune from complete extinction may lead eventually to a greater maturity in 
political and social thought than is generally in evidence now. 

As we discussed earlier, the human beings now living on the earth form 
a single polymorphic, polytypic species, Homo sapiens. The advent of more 
efficient transportation and the resulting greater ease of movement and contact 
among human groups have led to a breakdown in genetic isolates and an increase 
in gene flow among different human populations. Although this tendency has 
not resulted in the obliteration of racial differences, there can be little question 
that hybridization is a greater factor in human evolution now than at any time 
in the past. Furthermore, this situation seems likely to continue. 

Man's Future Numbers 

Another fairly safe prediction is that the human population will con- 
tinue to increase in numbers in the near future. Even safer is the prediction that 
this increase in population jize cannot continue unchecked indefinitely. Sooner 
or later death rates will equal birth rates, and population growth will cease. The 
significant question is whether the death rates will rise to match high birth rates, 
which would signalize a painful, tragic decline in standards of living, or whether 
they will equilibrate at a low level. Birth and death rates may seem to be crude 
indices of civilization, culture, or standards of living; nevertheless, they are at 
present very sensitive indicators of the status of a society. Man's future to a large 
extent will depend upon how successfully the human population adjusts to the 
available resources. Very few people accept a bare subsistence level as an ade- 
quate way of life, but if population expansion continues, this is the status that 
all mankind will eventually reach. Before they do, however, bitter and deva- 
stating conflicts seem inevitable. Since human population growth has been due to 
the dramatic reduction in the death rate, it is clear that generally acceptable 
means of controlling birth rates are essential if the population explosion is to be 
controlled before it leads to disaster. 

Homo sapiens is a dominant species because of the superior intelligence 
of its members. This mental ability made possible the development of culture; 
and cultural evolution, as distinct from biological evolution, has added a new 
dimension to the process of evolutionary change. It seems safe to predict that 



man's future • 375 

cultural progress will continue. One need only mention progress since the turn 
of the century in such fields as physics, aeronautics, genetics, and medicine, to 
emphasize what tremendous strides have been made. The end to this advance is 
not yet in sight. However, cultural evolution has not superceded biological evo- 
lution but has supplemented it. Biological evolution will continue in man, under 
the influence of the same evolutionary forces that have affected man as well as 
other species in the past. Modern medical discoveries have not eliminated the 
operation of selection in human populations; rather, the selection pressures have 
been modified or changed. The factors affecting reproductive fitness in modern 
society may be different from those operating in a primitive society, but there is 
no reason to suppose that selection has ceased to function altogether. 

Man's Genetic Future 

It seems probable that the human "load of mutations," the frequency of 
deleterious genes in the human population, will continue to increase in the near 
future. Because of their effects on mutation rates, the advent of the atomic age 
and the widespread use of mutagenic ionizing radiations in industry and in 
medical practice will be responsible in part for this increase. To the extent that 
medicine is successful in counteracting the harmful effects of deleterious genes 
so that affected individuals survive and reproduce, the frequencies of such genes 
will increase. It is not yet possible to predict just how serious the effects of these 
trends may be, but it hardly seems likely that they will be favorable. Rather, 
there will be a somewhat greater percentage of persons who by medical or other 
environmental manipulations must counteract the harmful effects of their genes. 

The question has been raised as to whether current trends are not lead- 
ing to a dissipation of the favorable genotypes of the past and to an increase in 
the frequency of deleterious or unfavorable genes in human populations. This 
question is a very fundamental one, for even though cultural or environmental 
remedies can to some extent compensate for genetic deficiencies, there must be a 
point beyond which such measures are inadequate. If too great a proportion of 
the population were to pass that point, any modern society would collapse. Lest 
you feel that this picture is an exaggeration, consider what would happen if a 
group of chimpanzees were made responsible for running a large city. No matter 
how carefully they were trained for their jobs from birth onward, chaos would 
result, for the tasks would be beyond the capacity of their genotypes even if they 
were all exceptionally able chimpanzees. Concern about the possible genetic 
deterioration of man has been expressed because so many factors at present seem 
to be favoring an increase in frequency of harmful genes in human populations. 
In addition to the increased load of mutations mentioned above, differential 
fertility in many countries leads to a disproportionate number of children being 
born to the parents least able to give them a favorable home environment and 



376 • EVOLUTION AND MAN 

least likely to endow them with a favorable genotype. In the United States, for 
example, one sixth of the women are now bringing one half of the children of 
the next generation into families with only one tenth of the national income. 
Since a laissez-faire policy seems likely to lead to a loss in genetic value, a 
number of eugenic programs have been proposed, aimed at the genetic betterment 
of mankind. Because of the radical nature of some of these proposals, especially 
by early proponents, and because the Nazi pogroms were carried out under the 
guise of a eugenics program, the term "eugenics" has come to have rather sinister 
connotations. The current arguments for the need for eugenic measures are based 
on the evidence that the net effect of many human activities is at present leading 
to a deterioration of the human gene pool. It is argued that we cannot afford to 
let this deterioration continue unchecked but must apply our present knowledge 
to human genetic improvement just as, through conscious effort, we have im- 
proved domesticated species of plants and animals. Two types of programs have 
been suggested: positive eugenic measures to increase the frequency of favorable 
genes and gene combinations, and negative eugenic measures to reduce the fre- 
quency of deleterious genes. All of these measures merit thoughtful considera- 
tion, but they also require careful scrutiny because of the risks inherent in any 
program of deliberate interference with human reproduction. 

Eugenics 

The great difficulty with any positive eugenics program is that decisions 
must be made as to which traits are to be favored. These decisions will be based 
on value judgments, for they cannot be made in any scientific manner. There- 
fore, the primary question becomes, whose set of values shall prevail, for it is 
unlikely that there would be any universal agreement sufficiently specific to per- 
mit setting up an effective program. Any program put into effect without uni- 
versal acceptance would represent an unwarranted infringement on human rights. 
Furthermore, it may even be an error to assume that human evolution should be 
guided toward any single goal or set of values. The genetic problems involved 
in breeding a new type of corn or hog are relatively simple. The measure of 
success is in the increased economic value of the product, but this is not the way 
we measure men. 

At present, negative eugenics seems more likely to be accepted because 
it is generally agreed that traits such as hereditary blindness, deafness, or similar 
severe afflictions are undesirable. For this reason it is possible through genetic 
counseling to convey to the persons concerned sufficient understanding of the 
hereditary risks involved so that they can make informed decisions concerning 
their own reproduction. Institutionalization of mentally defective or psychotic 
persons is a eugenic measure, since they do not ordinarily reproduce while insti- 
tutionalized. The usefulness of negative eugenics has sometimes been questioned 



man's future • 377 

on the grounds that its effect in reducing the frequency of recessive genes is so 
slight. However, from a humanitarian standpoint any action that averts the birth 
of a single afflicted person must be regarded as beneficial. 

The effectiveness of negative eugenics could be greatly enhanced if we 
had means to detect heterozygous carriers of deleterious recessive genes. Some 
traits can now be detected in heterozygotes, and it seems probable that as more 
refined techniques are discovered, additional information of this sort will become 
available. A quick reduction in the incidence of individuals affected by harmful 
dominant genes is already possible; detection of heterozygous carriers would 
make it possible to reduce still further the incidence of persons afflicted with re- 
cessive hereditary diseases. 

The success of such a program would depend upon the voluntary co- 
operation of a well-informed people and would have to be based on the universal 
desire of parents to have normal, healthy children. Any approach involving coer- 
cion could not be justified in a society that even pretended to be free. 

It may be argued that a program of such limited objectives is not ade- 
quate in the face of such threats to man's heritage as an increased load of muta- 
tions or differential fertility. However, we know very little about the magnitude 
or even the direction of the selection pressures operative in man at the present 
time. For example, it is well known that the average life span of married men is 
longer than that of bachelors, a statistic frequently cited as evidence for the bene- 
ficial effects of a life of wedded bliss. If one were to weigh all of the variables 
involved, one might conclude that the bachelors, rather than the married men, 
had every right to expect a longer life span. An alternative explanation for this 
fact is that women tend to marry the healthier men and that a selective process 
of considerable genetic significance, rather than an environmental effect, is re- 
sponsible for the difference in life span. A careful study would be necessary to 
determine which of these alternatives is correct. 

Another bit of data of possible significance is the fact that in the United 
States, on the average, only about 90 percent of all women past reproductive age 
have ever married. Furthermore, among such married women about 15 to 20 
percent have never had any children. Thus, the total reproductive burden is 
being carried by only three quarters of the women in any generation. There is 
no evidence whatever that there are any genetic differences between women who 
marry and those who do not, or between married women who have children and 
those who do not. However, the proportions involved are so great that if any 
genetic differentials are involved, they could be of considerable importance. 
Research to test these possibilities has yet to be carried out. Until these and other 
possibilities for positive selection pressure have been explored, the extent of the 
genetic deterioration of the human gene pool cannot be estimated with any 
degree of confidence. The great and obvious need is for more research, not just 
in medical genetics, but in all aspects of human genetics. 



378 • EVOLUTION AND MAN 

Where actions affecting human reproduction are already being taken, it 
is clear that some attention should be paid to their eugenic implications. Arti- 
ficial insemination, for example, is being done on an ever-wider scale, and here 
the responsibility for serious consideration of the genotype of the donor is clear. 
Furthermore, persons with a corrected or ameliorated genetic condition should 
certainly be made aware of the genetic risks involved in their reproduction and 
of their responsibility to future generations. The point is that as other medical, 
biological, and genetic techniques are discovered, they will unquestionably be 
used, and they will also undoubtedly affect the course of human evolution. The 
problem is to insure that these discoveries are used with wisdom and under- 
standing so that man's genetic heritage, certainly his most precious possession, is 
not needlessly frittered away. 

So much for man's future; what about future man himself? If still 
here, he will probably be somewhat different from us physically. If past trends 
continue, his head may well be larger than ours, with the face and teeth 
still further reduced. His personality may be such that we would consider him a 
genius, or perhaps a dolt, a criminal, or a crackpot, or even quite normal. 
Whether we would like him or not is of little consequence, for we shall never 
have to try to get along with him. 

SUGGESTED READING 

Haldane, J. B. S., 1949. "Human evolution: past and future," Genetics, paleontology 

and evolution. G. L. Jepsen, E. Mayr, and G. G. Simpson, eds. Princeton, 

N. J. : Princeton University Press. 
Muller, H. J., I960. "The guidance of human evolution," Evolution after Darwin, 

Vol. 2, The evolution of man, Sol Tax, ed. Chicago: University of Chicago 

Press. 
Osborn, F., 1951. Preface to eugenics. New York: Harper. 
Reed, S. C, 1955. Counseling in medical genetics. Philadelphia: Saunders. 



APPENDIX 



A 



From Charles Darwin's 
Voyage of the Beagle 



APPENDIX 



B 



From Thomas Malthus' 
Essay on the Principle 

of Population 4th edition 



APPENDIX 



A 



Chapter XVII— 
Galapagos Archipelago 



September 15th. — This archipelago consists of ten principal islands, of 
which five exceed the others in size. They are situated under the Equator, and 
between five and six hundred miles westward of the coast of America. They are 
all formed of volcanic rocks; a few fragments of granite curiously glazed and 
altered by the heat, can hardly be considered as an exception. Some of the 
craters, surmounting the larger islands, are of immense size, and they rise to a 
height of between three and four thousand feet. Their flanks are studded by 
innumerable smaller orifices. I scarcely hesitate to affirm, that there must be in 
the whole archipelago at least two thousand craters. These consist either of lava 
and scoriae, or of finely-stratified, sandstone-like tuff. Most of the latter are beau- 
tifully symmetrical; they owe their origin to eruptions of volcanic mud without 
any lava: it is a remarkable circumstance that every one of the twenty-eight tuff- 
craters which were examined, had their southern sides either much lower than 
the other sides, or quite broken down and removed. As all these craters appar- 
ently have been formed when standing in the sea, and as the waves from the 
trade wind and the swell from the open Pacific here unite their forces on the 
southern coasts of all the islands, this singular uniformity in the broken state of 
the craters, composed of the soft and yielding tuff, is easily explained. 

From Charles Darwin, 1887. Journal of researches into the natural history and geology 
of the countries visited during the voyage of H.M.S. Beagle round the world. New ed. 
New York: D. Appleton and Company. Pages 372-73, 377-81, 393-98. 

381 



382 • APPENDIX 

Considering that these islands are placed directly under the equator, the 
climate is far from being excessively hot; this seems chiefly caused by the singu- 
larly low temperature of the surrounding water, brought here by the great 
southern Polar current. Excepting during one short season, very little rain falls, 
and even then it is irregular; but the clouds generally hang low. Hence, whilst 
the lower parts of the islands are very sterile, the upper parts, at a height of a 
thousand feet and upwards, possess a damp climate and a tolerably luxuriant 
vegetation. This is especially the case on the windward sides of the islands, which 
first receive and condense the moisture from the atmosphere. . . . 

The natural history of these islands is eminently curious, and well de- 
serves attention. Most of the organic productions are aboriginal creations, found 
nowhere else; there is even a difference between the inhabitants of the different 
islands; yet all show a marked relationship with those of America, though sepa- 
rated from that continent by an open space of ocean, between 500 and 600 miles 
in width. The archipelago is a little world within itself, or rather a satellite at- 
tached to America, whence it has derived a few stray colonists, and has received 
the general character of its indigenous productions. Considering the small size of 
these islands, we feel the more astonished at the number of their aboriginal 
beings, and at their confined range. Seeing every height crowned with its crater, 
and the boundaries of most of the Java-streams still distinct, we are led to believe 
that within a period, geologically recent, the unbroken ocean was here spread 
out. Hence, both in space and time, we seem to be brought somewhat near to 
that great fact — that mystery of mysteries — the first appearance of new beings on 
this earth. 

Of terrestrial mammals, there is only one which must be considered as 
indigenous, namely, a mouse (Mus Galapagoensis), and this is confined, as far 
as I could ascertain, to Chatham island, the most easterly island of the group. It 
belongs, as I am informed by Mr. Waterhouse, to a division of the family of 
mice characteristic of America. At James island, there is a rat sufficiently distinct 
from the common kind to have been named and described by Mr. Waterhouse; 
but as it belongs to the old-world division of the family, and as this island has 
been frequented by ships for the last hundred and fifty years, I can hardly doubt 
that this rat is merely a variety, produced by the new and peculiar climate, food, 
and soil, to which it has been subjected. Although no one has a right to speculate 
without distinct facts, yet even with respect to the Chatham island mouse, it 
should be borne in mind, that it may possibly be an American species imported 
here; for I have seen, in a most unfrequented part of the Pampas, a native mouse 
living in the roof of a newly-built hovel, and therefore its transportation in a 
vessel is not improbable; analogous facts have been observed by Dr. Richardson 
in North America. 

Of land-birds I obtained twenty-six kinds, all peculiar to the group and 
found nowhere else, with the exception of one lark-like finch from North 
America (Dolichonyx oryzivorus), which ranges on that continent as far north 



APPENDIX • 383 

as 54°, and generally frequents marshes. The other twenty-five birds consist, 
firstly, of a hawk, curiously intermediate in structure between a Buzzard and the 
American group of carrion-feeding Polybori; and with these latter birds it agrees 
most closely in every habit and even tone of voice. Secondly, there are two owls, 
representing the short-eared and white barn-owls of Europe. Thirdly, a wren, 
three tyrant fly-catchers (two of them species of Pyocephalus, one or both of 
which would be ranked by some ornithologists as only varieties), and a dove — 
all analogous to, but distinct from, American species. Fourthly, a swallow, which 
though differing from the Progne purpurea of both Americas, only in being 
rather duller coloured, smaller, and slenderer, is considered by Mr. Gould as 
specifically distinct. Fifthly, there are three species of mocking-thrush — a form 
highly characteristic of America. The remaining land-birds form a most singular 
group of finches, related to each other in the structure of their beaks, short tails, 
form of body, and plumage: there are thirteen species, which Mr. Gould has 
divided into four sub-groups. All these species are peculiar to this archipelago; 
and so is the whole group, with the exception of one species of the sub-group 
Cactornis, lately brought from Bow island, in the Low Archipelago. Of Cactornis, 
the two species may be often seen climbing about the flowers of the great cactus- 
trees; but all the other species of this group of finches, mingled together in 
flocks, feed on the dry and sterile ground of the lower districts. The males of all, 
or certainly of the greater number, are jet black; and the females (with perhaps 
one or two exceptions) are brown. The most curious fact is the perfect gradation 
in the size of the beaks in the different species of Geospiza, from one as large as 
that of a hawfinch to that of a chaffinch, and (if Mr. Gould is right in including 
his sub-group, Certhidea. in the main group), even to that of a warbler. The 
largest beak in the genus Geospiza is shown in Fig. 1, and the smallest in Fig. 3; 
but instead of there being only one intermediate species, with a beak of the size 
shown in Fig. 2, there are no less than six species with insensibly graduated 
beaks. The beak of the sub-group Certhidea, is shown in Fig. 4. [Refer to 
text Fig. 31-4.] The beak of Cactornis is somewhat like that of a starling; and 
that of the fourth sub-group, Camarhynchus, is slightly parrot-shaped. Seeing 
this gradation and diversity of structure in one small, intimately related group 
of birds, one might really fancy that from an original paucity of birds in this 
archipelago, one species had been taken and modified for different ends. In a 
like manner it might be fancied that a bird originally a buzzard, had been in- 
duced here to undertake the office of the carrion-feeding Polybori of the Amer- 
ican continent. 

Of waders and water-birds I was able to get only eleven kinds, and of 
these only three (including a rail confined to the damp summits of the islands) 
are new species. Considering the wandering habits of the gulls, I was surprised to 
find that the species inhabiting these islands is peculiar, but allied to one from 
the southern parts of South America. The far greater peculiarity of the land- 
birds, namely, twenty-five out of twenty-six being new species or at least new 



384 • APPENDIX 

races, compared with the waders and web-footed birds, is in accordance with the 
greater range which these latter orders have in all parts of the world. We shall 
hereafter see this law of aquatic forms, whether marine or fresh-water, being 
less peculiar at any given point of the earth's surface than the terrestrial forms 
of the same classes, strikingly illustrated in the shells, and in a lesser degree in 
the insects of this archipelago. 

Two of the waders are rather smaller than the same species brought 
from other places: the swallow is also smaller, though it is doubtful whether or 
not it is distinct from its analogue. The two owls, the two tyrant fly-catchers 
(Pyrocephalus) and the dove, are also smaller than the analogous but distinct 
species, to which they are most nearly related; on the other hand, the gull is 
rather larger. The two owls, the swallow, all three species of mocking-thrush, 
the dove in its separate colours though not in its whole plumage, the Totanus, 
and the gull, are likewise duskier coloured than their analogous species; and in 
the case of the mocking-thrush and Totanus, than any other species of the two 
genera. With the exception of a wren with a fine yellow breast, and of a tyrant 
fly-catcher with a scarlet tuft and breast, none of the birds are brilliantly 
coloured, as might have been expected in an equatorial district. Hence it would 
appear probable, that the same causes which here make the immigrants of some 
species smaller, make most of the peculiar Galapageian species also smaller, as 
well as very generally more dusky coloured. All the plants have a wretched, 
weedy appearance, and I did not see one beautiful flower. The insects, again, are 
small sized and dull coloured, and, as Mr. Waterhouse informs me, there is 
nothing in their general appearance which would have led him to imagine that 
they had come from under the equator. The birds, plants, and insects have a 
desert character, and are not more brilliantly coloured than those from southern 
Patagonia; we may, therefore, conclude that the usual gaudy colouring of the 
intertropical productions, is not related either to the heat or light of those zones, 
but to some other cause, perhaps to the conditions of existence being generally 
favourable to life. 

. . . Dr. Hooker informs me that the Flora has an undoubted Western 
American character; nor can he detect in it any affinity with that of the Pacific. 
If, therefore, we except the eighteen marine, the one fresh-water, and one land- 
shell, which have apparently come here as colonists from the central islands of 
the Pacific, and likewise the one distinct Pacific species of the Galapageian group 
of finches, we see that this archipelago, though standing in the Pacific Ocean, is 
zoologically part of America. 

If this character were owing merely to immigrants from America, there 
would be little remarkable in it; but we see that a vast majority of all the land 
animals, and that more than half of the flowering plants, are aboriginal produc- 
tions. It was most striking to be surrounded by new birds, new reptiles, new 
shells, new insects, new plants, and yet by innumerable trifling details of struc- 



APPENDIX • 385 

ture, and even by the tones of voice and plumage of the birds, to have the 
temperate plains of Patagonia, or the hot dry deserts of Northern Chile, vividly 
brought before my eyes. Why, on these small points of land, which within a late 
geological period must have been covered by the ocean, which are formed of 
basaltic lava, and therefore differ in geological character from the American 
continent, and which are placed under a peculiar climate, — why were their 
aboriginal inhabitants, associated, I may add, in different proportions both in 
kind and number from those on the continent, and therefore acting on each 
other in a different manner — why were they created on American types of 
organization? It is probable that the islands of the Cape de Verd group resem- 
ble, in all their physical conditions, far more closely the Galapagos Islands than 
these latter physically resemble the coast of America; yet the aboriginal inhab- 
itants of the two groups are totally unlike; those of the Cape de Verd Islands 
bearing the impress of Africa, as the inhabitants of the Galapagos Archipelago 
are stamped with that of America. 

I have not as yet noticed by far the most remarkable feature in the 
natural history of this archipelago; it is, that the different islands to a consider- 
able extent are inhabited by a different set of beings. My attention was first called 
to this fact by the Vice-Governor, Mr. Lawson, declaring that the tortoises dif- 
fered from the different islands, and that he could with certainty tell from which 
island any one was brought. I did not for some time pay sufficient attention to 
this statement, and I had already partially mingled together the collections from 
two of the islands. I never dreamed that islands, about fifty or sixty miles apart, 
and most of them in sight of each other, formed of precisely the same rocks, 
placed under a quite similar climate, rising to a nearly equal height, would have 
been differently tenanted; but we shall soon see that this is the case. It is the fate 
of most voyagers, no sooner to discover what is most interesting in any locality, 
than they are hurried from it; but I ought, perhaps, to be thankful that I ob- 
tained sufficient materials to establish this most remarkable fact in the distribu- 
tion of organic beings. 

The inhabitants, as I have said, state that they can distinguish the tor- 
toises from the different islands; and that they differ not only in size but in 
other characters. Captain Porter has described those from Charles and from the 
nearest island to it, namely, Hood Island, as having their shells in front thick 
and turned up like a Spanish saddle, whilst the tortoises from James Island are 
rounder, blacker, and have a better taste when cooked. M. Bibron, moreover, 
informs me that he has seen what he considers two distinct species of tortoise 
from the Galapagos, but he does not know from which islands. The specimens 
that I brought from three islands were young ones; and probably owing to this 
cause, neither Mr. Gray nor myself could find in them any specific differences. 
I have remarked that the marine Amblyrhynchus was larger at Albemarle Island 
than elsewhere; and M. Bibron informs me that he has seen two distinct aquatic 
species of this genus; so that the different islands probably have their representa- 



386 • APPENDIX 

tive species or races of the Amblyrhynchus, as well as of the tortoise. My 
attention was first thoroughly aroused, by comparing together the numerous spe- 
cimens, shot by myself and several other parties on board, of the mocking- 
thrushes, when, to my astonishment, I discovered that all those from Charles 
Island belonged to one species (Mimus trifasciatus) ; all from Albemarle Island 
to M. parvulus; and all from James and Chatham Islands (between which two 
other islands are situated, as connecting links) belonged to M. melanotis. These 
two latter species are closely allied, and would by some ornithologists be con- 
sidered as only well-marked races or varieties; but the Mimus trifasciatus is very 
distinct. Unfortunately most of the specimens of the finch tribe were mingled 
together; but I have strong reasons to suspect that some of the species of the 
sub-group Geospiza are confined to separate islands. If the different islands have 
their representatives of Geospiza, it may help to explain the singularly large 
number of the species of this sub-group in this one small archipelago, and as a 
probable consequence of their numbers, the perfectly graduated series in the 
size of their beaks. Two species of the sub-group Cactornis, and two of 
Camarhynchus, were procured in the archipelago; and of the numerous spe- 
cimens of these two sub-groups shot by four collectors at James Island, all were 
found to belong to one species of each; whereas the numerous specimens shot 
either on Chatham or Charles Island (for the two sets were mingled together) 
all belonged to the two other species: hence we may feel almost sure that these 
islands possess their representative species of these two sub-groups. In land- 
shells this law of distribution does not appear to hold good. In my very small 
collection of insects, Mr. Waterhouse remarks, that of those which were ticketed 
with their locality, not one was common to any two of the islands. 

If we now turn to the Flora, we shall find the aboriginal plants of the 
different islands wonderfully different. I give all the following results on the 
high authority of my friend Dr. J. Hooker. I may premise that I indiscriminately 
collected everything in flower on the different islands, and fortunately kept my 
collections separate. Too much confidence, however, must not be placed in the 
proportional results, as the small collections brought home by some other natural- 
ists, though in some respects confirming the results, plainly show that much re- 
mains to be done in the botany of this group: the Leguminosae, moreover, have 
as yet been only approximately worked out. [See table on next page, Ed.] 

Hence we have the truly wonderful fact, that in James Island, of the 
thirty-eight Galapageian plants, or those found in no other part of the world, 
thirty are exclusively confined to this one island; and in Albemarle Island, of the 
twenty-six aboriginal Galapageian plants, twenty-two are confined to this one 
island, that is, only four are at present known to grow in the other islands of the 
archipelago; and so on, as shown in the table [below], with the plants from 
Chatham and Charles Islands. This fact will, perhaps, be rendered even more 
striking, by giving a few illustrations: — thus, Scalesia, a remarkable arborescent 
genus of the Compositae, is confined to the archipelago: it has six species; one 



APPENDIX • 387 













No. of Species 






No. of 


No. of 




confined to the 






Species 


Species 


No. 


Galapagos 


Name 


Total 


found in 


confined 


confined 


Archipelago, 


of 


No. of 


other parts 
of the 


to the 


to the 


but found on 


Island 


Species 


Galapagos 


one 


more than the 






world 


Archipelago 


Island 


one Island 


James Island 


71 


33 


38 


30 


8 


Albemarle Island 


46 


18 


26 


22 


4 


Chatham Island 


32 


16 


16 


12 


4 


Charles Island 


68 


39 
(or 29, if 

the probably- 
imported 
plants be 

subtracted) 


29 


21 


8 



from Chatham, one from Albemarle, one from Charles Island, two from James 
Island, and the sixth from one of the three latter islands, but it is not known 
from which: not one of these six species grows on any two islands. Again, 
Euphorbia, a mundane or widely distributed genus, has here eight species, of 
which seven are confined to the archipelago, and not one found on any two 
islands: Acalypha and Borreria, both mundane genera, have respectively six and 
seven species, none of which have the same species on two islands, with the 
exception of one Borreria, which does occur on two islands. The species of the 
Compositae are particularly local; and Dr. Hooker has furnished me with several 
other most striking illustrations of the difference of the species on the different 
islands. He remarks that this law of distribution holds good both with those 
genera confined to the archipelago, and those distributed in other quarters of the 
world: in like manner we have seen that the different islands have their proper 
species of the mundane genus of tortoise, and of the widely distributed Amer- 
ican genus of the mocking-thrush, as well as of two of the Galapageian sub- 
groups of finches, and almost certainly of the Galapageian genus Amblyrhynchus. 
The distribution of the tenants of this archipelago would not be nearly 
so wonderful, if, for instance, one island had a mocking-thrush, and a second 
island some other quite distinct genus; — if one island had its genus of lizard, 
and a second island another distinct genus, or none whatever; — or if the dif- 
ferent islands were inhabited, not by representative species of the same genera of 
plants, but by totally different genera, as does to a certain extent hold good; for, 
to give one instance, a large berry-bearing tree at James Island has no representa- 
tive species in Charles Island. But it is the circumstance, that several of the 
islands possess their own species of the tortoise, mocking-thrush, finches, and 



388 * APPENDIX 

numerous plants, these species having the same general habits, occupying analo- 
gous situations, and obviously filling the same place in the natural economy of 
this archipelago, that strikes me with wonder. It may be suspected that some of 
these representative species, at least in the case of the tortoise and of some of the 
birds, may hereafter prove to be only well-marked races; but this would be of 
equally great interest to the philosophical naturalist. I have said that most of the 
islands are in sight of each other: I may specify that Charles Island is fifty miles 
from the nearest part of Chatham Island, and thirty-three miles from the nearest 
part of Albemarle Island. Chatham Island is sixty miles from the nearest part of 
James Island, but there are two intermediate islands between them which were 
not visited by me. James Island is only ten miles from the nearest part of Albe- 
marle Island, but the two points where the collections were made are thirty-two 
miles apart. I must repeat, that neither the nature of the soil, nor height of the 
land, nor the climate, nor the general character of the associated beings, and 
therefore their action one on another, can differ much in the different islands. 
If there be any sensible difference in their climates, it must be between the wind- 
ward group (namely Charles and Chatham Islands), and that to leeward; but 
there seems to be no corresponding difference in the productions of these two 
halves of the archipelago. 

The only light which I can throw on this remarkable difference in the in- 
habitants of the different islands, is, that very strong currents of the sea running 
in a westerly and W.N.W. direction must separate, as far as transportal by the sea 
is concerned, the southern islands from the northern ones; and between these 
northern islands a strong N.W. current was observed, which must effectually 
separate James and Albemarle Islands. As the archipelago is free to a most re- 
markable degree from gales of wind, neither the birds, insects, nor lighter seeds, 
would be blown from island to island. And lastly, the profound depth of the 
ocean between the islands, and their apparently recent (in a geological sense) 
volcanic origin, render it highly unlikely that they were ever united; and this, 
probably, is a far more important consideration than any other, with respect to 
the geographical distribution of their inhabitants. Reviewing the facts here given, 
one is astonished at the amount of creative force, if such an expression may be 
used, displayed on these small, barren, and rocky islands; and still more so, at its 
diverse yet analogous action on points so near each other. I have said that the 
Galapagos Archipelago might be called a satellite attached to America, but it 
should rather be called a group of satellites, physically similar, organically dis- 
tinct, yet intimately related to each other, and all related in a marked, though 
much lesser degree, to the great American continent. 



APPENDIX 



B 



An Essay on the Principle 

of Population, Book I 



Of the Checks to Population in the Less Civilized 
Parts of the World and in Past Times 

Chapter 1 — statement of the subject. 

RATIOS OF THE INCREASE OF POPULATION 
AND FOOD 

In an inquiry concerning the improvement of society, the mode of conducting 
the subject which naturally presents itself, is 

1. To investigate the causes that have hitherto impeded the progress of 
mankind towards happiness; and 

2. To examine the probability of the total or partial removal of these 
causes in future. 

To enter fully into this question, and to enumerate all the causes that 
have hitherto influenced human improvement, would be much beyond the power 
of an individual. The principle object of the present essay is to examine the 
effects of one great cause intimately united with the very nature of man; which, 

From T. R. Malthus, 1807. An essay on the principle of population. Fourth ed. London: 
J. Johnson in St. Paul's Churchyard. Chapters 1 and 2. 

389 



390 • APPENDIX 

though it has been constantly and powerfully operating since the commencement 
of society, has been little noticed by the writers who have treated this subject. 
The facts which establish the existence of this cause have, indeed, been repeatedly 
stated and acknowledged; but its natural and necessary effects have been almost 
totally overlooked; though probably among these effects may be reckoned a very 
considerable portion of that vice and misery, and of that unequal distribution of 
the bounties of nature, which it has been the unceasing object of the enlightened 
philanthropist in all ages to correct. 

The cause to which I allude, is the constant tendency in all animated 
life to increase beyond the nourishment prepared for it. 

It is observed by Dr. Franklin, that there is no bound to the prolific 
nature of plants or animals, but what is made by their crowding and interfering 
with each others means of subsistence. Were the face of the earth, he says, vacant 
of other plants, it might be gradually sowed and overspread with one kind only, 
as for instance with fennel : and were it empty of other inhabitants, it might in 
a few ages be replenished from one nation only, as for instance with Englishmen. 

This is incontrovertibly true. Through the animal and vegetable king- 
doms Nature has scattered the seeds of life abroad with the most profuse and 
liberal hand; but has been comparatively sparing in the room and the nourish- 
ment necessary to rear them. The germs of existence contained in this earth, if 
they could freely develope themselves, would fill millions of worlds in the course 
of a few thousand years. Necessity, that imperious all-pervading law of nature, 
restrains them within the prescribed bounds. The race of plants and the race of 
animals shrink under this great restrictive law; and man cannot by any efforts of 
reason escape from it. 

In plants and irrational animals, the view of the subject is simple. 
They are all impelled by a powerful instinct to the increase of their species; and 
this instinct is interrupted by no doubts about providing for their offspring. 
Wherever therefore there is liberty, the power of increase is exerted; and the 
superabundant effects are repressed afterwards by want of room and nourishment. 

The effects of this check on man are more complicated. Impelled to the 
increase of his species by an equally powerful instinct, reason interrupts his 
career, and asks him whether he may not bring beings into the world, for whom 
he cannot provide the means of support. If he attend to this natural suggestion, 
the restriction too frequently produces vice. If he hear it not, the human race 
will be constantly endeavoring to increase beyond the means of subsistence. But 
as by that law of our nature which makes food necessary to the life of man, 
population can never actually increase beyond the lowest nourishment capable 
of supporting it, a strong check on population, from the difficulty of acquiring 
food, must be constantly in operation. This difficulty must fall somewhere, and 
must necessarily be severely felt in some or other of the various forms of misery, 
or the fear of misery, by a large portion of mankind. 

That population has this constant tendency to increase beyond the 



APPENDIX • 391 

means of subsistence, and that it is kept to its necessary level by these causes, 
will sufficiently appear from a review of the different states of society in which 
man has existed. But before we proceed to this review, the subject will perhaps 
be seen in a clearer light, if we endeavour to ascertain, what would be the 
natural increase of population, if left to exert itself with perfect freedom; and 
what might be expected to be the rate of increase in the productions of the 
earth, under the most favourable circumstances of human industry. 

It will be allowed, that no country has hitherto been known, where the 
manners were so pure and simple, and the means of subsistence so abundant, 
that no check whatever has existed to early marriages from the difficulty of pro- 
viding for a family, and that no waste of the human species has been occasioned 
by vicious customs, by towns, by unhealthy occupations, or too severe labour. 
Consequently in no state that we have yet known, has the power of population 
been left to exert itself with perfect freedom. 

Whether the law of marriage be instituted, or not, the dictate of nature 
and virtue seems to be an early attachment to one woman; and where there were 
no impediments of any kind in the way of an union to which such an attachment 
would lead, and no causes of depopulation afterwards, the increase of the human 
species would be evidently much greater than any increase which has been 
hitherto known. 

In the northern states of America, where the means of subsistence have 
been more ample, the manners of the people more pure, and the checks to early 
marriages fewer, than in any of the modern states of Europe, the population has 
been found to double itself, for above a century and a half successively, in less 
than in each period of twenty-five years. Yet even during these periods, in some 
of the towns, the deaths exceeded the births, a circumstance which clearly proves 
that in those parts of the country which supplied this deficiency, the increase 
must have been much more rapid than the general average. 

In the back settlements, where the sole employment is agriculture, and 
vicious customs and unwholesome occupations are little known, the population 
has been found to double itself in fifteen years. Even this extraordinary rate of 
increase is probably short of the utmost power of population. Very severe labour 
is requisite to clear a fresh country; such situations are not in general considered 
as particularly healthy; and the inhabitants are probably occasionally subject to 
the incursions of the Indians, which may destroy some lives, or at any rate 
diminish the fruits of their industry. 

According to a table of Euler, calculated on a mortality of 1 in 36, if 
the births be to the deaths in the proportion of 3 to 1, the period of doubling 
will be only 12% years. And this proportion is not only a possible supposition, 
but has actually occurred for short periods in more countries than one. 

Sir William Petty supposes a doubling possible in so short a time as 
ten years. 

But to be perfectly sure that we are far within the truth, we will take 



392 • APPENDIX 

the slowest of these rates of increase, a rate, in which all concurring testimonies 
agree, and which has been repeatedly ascertained to be from procreation only. 

It may safely be pronounced, therefore, that population, when un- 
checked, goes on doubling itself every twenty-five years, or increases in a geo- 
metrical ratio. 

The rate according to which the productions of the earth may be sup- 
posed to increase, it will not be so easy to determine. Of this, however, we may 
be perfectly certain, that the ratio of their increase must be totally of a different 
nature from the ratio of the increase of population. A thousand millions are just 
as easily doubled every twenty-five years by the power of population as a thou- 
sand. But the food to support the increase from the greater number will by no 
means be obtained with the same facility. Man is necessarily confined in room. 
When acre has been added to acre till all the fertile land is occupied, the yearly 
increase of food must depend upon the melioration of the land already in pos- 
session. This is a stream, which from the nature of all soils, instead of increasing, 
must be gradually diminishing. But population, could it be supplied with food, 
would go on with unexhausted vigour; and the increase of one period would 
furnish the power of a greater increase the next, and this without any limit. 

From the accounts we have of China and Japan, it may be fairly 
doubted, whether the best directed efforts of human industry could double the 
produce of these countries even once in any number of years. There are many 
parts of the globe, indeed, hitherto uncultivated, and almost unoccupied; but the 
right of exterminating, or driving into a corner where they must starve, even the 
inhabitants of these thinly populated regions, will be questioned in a moral view. 
The process of improving their minds and directing their industry would neces- 
sarily be slow; and during this time, as population would regularly keep pace 
with the increasing produce, it would rarely happen that a great degree of knowl- 
edge and industry would have to operate at once upon rich unappropriated soil. 
Even where this might take place, as it does sometimes in new colonies, a 
geometrical ratio increases with such extraordinary rapidity, that the advantage 
could not last long. If America continue increasing, which she certainly will do, 
though not with the same rapidity as formerly, the Indians will be driven further 
and further back into the country, till the whole race is ultimately exterminated. 

These observations are, in a degree, applicable to all the parts of the 
earth, where the soil is imperfectly cultivated. To exterminate the inhabitants of 
the greatest part of Asia and Africa, is a thought that could not be admitted for 
a moment. To civilize and direct the industry of the various tribes of Tartars and 
Negroes, would certainly be a work of considerable time, and of variable and 
uncertain success. 

Europe is by no means so fully peopled as it might be. In Europe there 
is the fairest chance that human industry may receive its best direction. The 
science of agriculture has been much studied in England and Scotland; and 



APPENDIX • 393 

there is still a great portion of uncultivated land in these countries. Let us con- 
sider, at what rate the produce of this island might be supposed to increase under 
circumstances the most favourable to improvement. 

If it be allowed, that by the best possible policy, and great encourage- 
ments to agriculture, the average produce of the island could be doubled in the 
first twenty-five years, it will be allowing probably a greater increase than could 
with reason be expected. 

In the next twenty-five years, it is impossible to suppose that the pro- 
duce could be quadrupled. It would be contrary to all our knowledge of the 
properties of land. The improvement of the barren parts would be a work of 
time and labour; and it must be evident to those who have the slightest ac- 
quaintance with agricultural subjects, that in proportion as cultivation extended, 
the additions that could yearly be made to the former average produce must be 
gradually and regularly diminishing. That we may be the better able to compare 
the increase of population and food, let us make a supposition, which, without 
pretending to accuracy, is clearly more favourable to the power of production in 
the earth, than any experience we have had of its qualities will warrant. 

Let us suppose that the yearly additions which might be made to the 
former average produce, instead of decreasing, which they certainly would do, 
were to remain the same; and that the produce of this island might be increased 
every twenty-five years, by a quantity equal to what it at present produces. The 
most enthusiastic speculator cannot suppose a greater increase than this. In a few 
centuries it would make every acre of land in the island like a garden. 

If this supposition be applied to the whole earth, and if it be allowed 
that the subsistence for man which the earth affords, might be increased every 
twenty-five years by a quantity equal to what it at present produces, this will be 
supposing a rate of increase much greater than we can imagine that any possible 
exertions of mankind could make it. 

It may be fairly pronounced therefore, that, considering the present 
average state of the earth, the means of subsistence, under circumstances the 
most favourable to human industry, could not possibly be made to increase faster 
than in an arithmetical ratio. 

The necessary effects of these two different rates of increase, when 
brought together, will be very striking. Let us call the population of this island 
eleven millions; and suppose the present produce equal to the easy support of 
such a number. In the first twenty-five years the population would be twenty-two 
millions, and the food being also doubled, the means of subsistence would be 
equal to this increase. In the next twenty-five years, the population would be 
forty-four millions, and the means of subsistence only equal to the support of 
thirty-three millions. In the next period the population would be eighty-eight 
millions, and the means of subsistence just equal to the support of half of that 
number. And at the conclusion of the first century, the population would be a 



394 • APPENDIX 

hundred and seventy-six millions, and the means of subsistence only equal to the 
support of fifty-five millions, leaving a population of a hundred and twenty-one 
millions totally unprovided for. 

Taking the whole earth instead of this island, emigration would of 
course be excluded; and supposing the present population equal to a thousand 
millions, the human species would increase as the numbers 1,2,4,8,16,32,64,128, 
256, and subsistence as 1,2,3,4,5,6,7,8,9. In two centuries the population would 
be to the means of subsistence as 256 to 9; in three centuries as 4096 to 13, and 
in two thousand years the difference would be almost incalculable. 

In this supposition no limits whatever are placed to the produce of the 
earth. It may increase for ever, and be greater than any assignable quantity; yet 
still the power of population being in every period so much superior, the increase 
of the human species can only be kept down to the level of the means of sub- 
sistence by the constant operation of the strong law of necessity acting as a check 
upon the greater power. 

Chapter 2 — of the general checks to 

POPULATION, AND THE MODE OF THEIR 
OPERATION 

The ultimate check to population appears then to be a want of food arising 
necessarily from the different ratios according to which population and food 
increase. But this ultimate check is never the immediate check, except in cases of 
actual famine. 

The immediate check may be stated to consist in all those customs, and 
all those diseases which seem to be generated by a scarcity of the means of sub- 
sistence; and all those causes, independent of this scarcity, whether of a moral or 
physical nature, which tend prematurely to weaken and destroy the human frame. 

These checks to population, which are constantly operating with more 
or less force in every society, and keep down the number to the level of the 
means of subsistence, may be classed under two general heads, the preventive, 
and the positive checks. 

The preventive check, as far as it is voluntary, is peculiar to man, and 
arises from that distinctive superiority in his reasoning faculties, which enables 
him to calculate distant consequences. The checks to the indefinite increase of 
plants and irrational animals are all either positive, or, if preventive, involuntary. 
But man cannot look around him, and see the distress which frequently presses 
upon those who have large families; he cannot contemplate his present posses- 
sions or earnings, which he now nearly consumes himself, and calculate the 
amount of each share, when with very little addition they must be divided, per- 
haps, among seven or eight, without feeling a doubt, whether if he follow the 
bent of his inclinations, he may be able to support the offspring which he will 
probably bring into the world. In a state of equality, if such can exist, this would 



APPENDIX • 395 

be a simple question. In the present state of society other considerations occur. 
Will he not lower his rank in life, and be obliged to give up in great measure 
his former habits? Does any mode of employment present itself by which he 
may reasonably hope to maintain a family? Will he not at any rate subject him- 
self to greater difficulties, and more severe labour than in his single state? Will 
he not be unable to transmit to his children the same advantages of education and 
improvement that he had himself possessed? Does he even feel secure that, 
should he have a large family, his utmost exertions can save them from rags and 
squalid poverty, and their consequent degradation in the community? And may 
he not be reduced to the grating necessity of forfeiting his independence, and of 
being obliged to the sparing hand of charity for support ? 

These considerations are calculated to prevent, and certainly do prevent, 
a great number of persons in all civilized nations from pursuing the dictate of 
nature in an early attachment to one woman. 

If this restraint do not produce vice, it is undoubtedly the least evil that 
can arise from the principle of population. Considered as a restraint on a strong 
natural inclination, it must be allowed to produce a certain degree of temporary 
unhappiness; but evidently slight, compared with the evils which result from 
any of the other checks to population; and merely of the same nature as many 
other sacrifices of temporary to permanent gratification, which it is the business 
of a moral agent continually to make. 

When this restraint produces vice, the evils which follow are but too 
conspicuous. A promiscuous intercourse to such a degree as to prevent the birth 
of children seems to lower in the most marked manner the dignity of human 
nature. It cannot be without its effect on men, and nothing can be more obvious 
than its tendency to degrade the female character, and to destroy all its most 
amiable and distinguishing characteristics. Add to which, that among those un- 
fortunate females with which all great towns abound, more real distress and 
aggravated misery are perhaps to be found, than in any other department of 
human life. 

When a general corruption of morals with regard to the sex pervades 
all the classes of society, its effects must necessarily be, to poison the springs of 
domestic happiness, to weaken conjugal and parental affection, and to lessen the 
united exertions and ardour of parents in the care and education of their chil- 
dren; effects which cannot take place without a decided diminution of the gen- 
eral happiness and virtue of the society; particularly as the necessity of art in the 
accomplishment and conduct of intrigues, and in the concealment of their conse- 
quences, necessarily leads to many other vices. 

The positive checks to population are extremely various, and include 
every cause, whether arising from vice or misery, which in any degree contributes 
to shorten the natural duration of human life. Under this head therefore may be 
enumerated all unwholesome occupations, severe labour and exposure to the 
seasons, extreme poverty, bad nursing of children, great towns, excesses of all 



396 • APPENDIX 

kinds, the whole train of common diseases and epidemics, wars, plagues, and 
famine. 

On examining these obstacles to the increase of population which I have 
classed under the heads of preventive and positive checks, it will appear that they 
are all resolvable into moral restraint, vice, and misery. 

Of the preventive checks, the restraint from marriage which is not fol- 
lowed by irregular gratifications may properly be termed moral restraint. Promis- 
cuous intercourse, unnatural passions, violations of the marriage bed, and im- 
proper arts to conceal the consequences of irregular connexions, are preventive 
checks that clearly come under the head of vice. 

Of the positive checks, those which appear to arise unavoidably from 
the laws of nature may be called exclusively misery; and those which we obvi- 
ously bring upon ourselves, such as wars, excesses, and many others which it 
would be in our power to avoid, are of a mixed nature. They are brought upon 
us by vice, and their consequences are misery. 

The sum of all these preventive and positive checks taken together 
forms the immediate check to population; and it is evident that in every country 
where the whole of the procreative power cannot be called into action, the pre- 
ventive and the positive checks must vary inversely as each other; that is, in 
countries either naturally unhealthy, or subject to a great mortality, from what- 
ever cause it may arise, the preventive check will prevail very little. In those 
countries, on the contrary, which are naturally healthy, and where the preventive 
check is found to prevail with considerable force, the positive check will prevail 
very little, or the mortality be very small. 

In every country some of these checks are, with more or less force, in 
constant operation; yet notwithstanding their general prevalence, there are few 
states in which there is not a constant effort in the population to increase beyond 
the means of subsistence. This constant effort as constantly tends to subject the 
lower classes of society to distress, and to prevent any great permanent meliora- 
tion of their condition. 

These effects, in the present state of society, seem to be produced in the 
following manner. We will suppose the means of subsistence in any country just 
equal to the easy support of its inhabitants. The constant effort towards popula- 
tion, which is found to act even in the most vicious societies, increases the num- 
ber of people before the means of subsistence are increased. The food therefore 
which before supported eleven millions, must now be divided among eleven mil- 
lions and a half. The poor consequently must live much worse, and many of 
them be reduced to severe distress. The number of labourers also being above 
the proportion of work in the market, the price of labour must tend to fall, 
while the price of provisions would at the same time tend to rise. The labourer 
therefore must do more work, to earn the same as he did before. During this 
season of distress the discouragements to marriage, and the difficulty of rearing 
a family are so great, that population is nearly at a stand. In the mean time, the 



APPENDIX • 397 

cheapness of labour, the plenty of labourers, and the necessity of an increased 
industry among them, encourage cultivators to employ more labour upon their 
land, to turn up fresh soil, and to manure and improve more completely what is 
already in tillage; till ultimately the means of subsistence may become in the 
same proportion to the population, as at the period from which we set out. The 
situation of the labourer being then again tolerably comfortable, the restraints to 
population are in some degree loosened; and, after a short period, the same 
retrograde and progressive movements, with respect to happiness, are repeated. 

This sort of oscillation will not probably be obvious to common view; 
and it may be difficult even for the most attentive observer to calculate its 
periods. Yet that in the generality of old states, some such vibration does exist, 
though in a much less marked, and in a much more irregular manner, than I 
have described it, no reflecting man who considers the subject deeply can well 
doubt. 

One principal reason why this oscillation has been less remarked, and 
less decidedly confirmed by experience than might naturally be expected, is, that 
the histories of mankind which we possess are, in general, histories only of the 
higher classes. We have not many accounts, that can be depended on, of the 
manners and customs of that part of mankind, where these retrograde and pro- 
gressive movements chiefly take place. A satisfactory history of this kind, of one 
people and of one period, would require the constant and minute attention of 
many observing minds in local and general remarks on the state of the lower 
class of society, and the causes that influenced it; and to draw accurate inferences 
upon this subject, a succession of such historians for some centuries would be 
necessary. This branch of statistical knowledge has of late years been attended to 
in some countries, and we may promise ourselves a clearer insight into the in- 
ternal structure of human society from the progress of these inquiries. But the 
science may be said yet to be in its infancy, and many of the objects, on which 
it would be desirable to have information, have either been omitted or not stated 
with sufficient accuracy. Among these perhaps may be reckoned, the proportion 
of the number of adults to the number of marriages; the extent to which vicious 
customs have prevailed in consequence of the restraints upon matrimony; the 
comparative mortality among the children of the most distressed part of the 
community, and of those who live rather more at their ease; the variations in the 
real price of labour; the observable differences in the state of the lower classes 
of society with respect to ease and happiness, at different times during a certain 
period; and very accurate registers of births, deaths, and marriages, which are of 
the utmost importance in this subject. 

A faithful history, including such particulars, would tend greatly to 
elucidate the manner in which the constant check upon population acts; and 
would probably prove the existence of the retrograde and progressive movements 
that have been mentioned; though the times of their vibration must necessarily 
be rendered irregular from the operation of many interrupting causes; such as, 



398 • APPENDIX 

the introduction of or failure of certain manufactures, a greater or less prevalent 
spirit of agricultural enterprise; years of plenty, or years of scarcity; wars, sickly 
seasons, poor laws, emigration, and other causes of a similar nature. 

A circumstance which has perhaps more than any other contributed to 
conceal this oscillation from common view is, the difference between the nominal 
and real price of labour. It very rarely happens that the nominal price of labour 
universally falls; but we well know that it frequently remains the same, while 
the nominal price of provisions has been gradually rising. This is, in effect, a 
real fall in the price of labour; and, during this period, the condition of the 
lower classes of the community must be gradually growing worse. But the farm- 
ers and capitalists are growing rich from the real cheapness of labour. Their 
increasing capitals enable them to employ a greater number of men; and, as the 
population had probably suffered some check from the greater difficulty of sup- 
porting a family, the demand for labour, after a certain period, would be great 
in proportion to the supply, and its price would of course rise, if left to find its 
natural level; and thus the wages of labour, and consequently the condition of 
the lower classes of society, might have progressive and retrograde movements, 
though the price of labour might never nominally fall. 

In savage life, where there is no regular price of labour, it is little to 
be doubted that similar oscillations take place. When population has increased 
nearly to the utmost limits of the food, all the preventive and the positive checks 
will naturally operate with increased force. Vicious habits with respect to the sex 
will be more general, the exposing of children more frequent, and both the 
probability and fatality of wars and epidemics will be considerably greater; and 
these causes will probably continue their operation till the population is sunk 
below the level of the food; and then the return to comparative plenty will again 
produce an increase, and, after a certain period, its further progress will again 
be checked by the same causes. 

But without attempting to establish these progressive and retrograde 
movements in different countries, which would evidently require more minute 
histories than we possess, and which the progress of civilization naturally tends 
to counteract, the following propositions are intended to be proved : 

1. Population is necessarily limited by the means of subsistence. 

2. Population invariably increases, where the means of subsistence in- 
crease, unless prevented by some very powerful and obvious checks. 

3. These checks, and the checks which repress the superior power of 
population, and keep its effects on a level with the means of subsistence, are all 
resolvable into moral restraint, vice, and misery. 

The first of these propositions scarcely needs illustration. The second 
and third will be sufficiently established by a review of the immediate checks to 
population in the past and present state of society. . . . 



Glossary 



Acentric — lacking a centromere. 

Adaptation — adjustment to environmental conditions by an organism or a popula- 
tion so that it becomes more fit for existence under the prevailing con- 
ditions. 

Adaptive radiation — the evolution from a common ancestry of morphologically and 
ecologically divergent types. 

Allele — one of a pair or series of alternative forms of a gene, occupying the same 
locus in homologous chromosomes. 

Allesthetic — traits that assume adaptive significance via the sense organs and nerv- 
ous system of other organisms. 

Allopatric — individuals or populations spatially isolated from one another. 

Allopolyploid — an organism with more than two sets of chromosomes derived from 
two or more species by hybridization. At meiosis, synapsis is primarily be- 
tween homologous chromosomes of like origin. 

Ammonites — an extinct group of mollusks related to the living chambered nautilus. 

Amphiploid — an allopolyploid. 

Analogous — similar in function but different in structure and origin. 

Anaphase — the stage in nuclear division during which the daughter chromosomes 
separate and move from the equatorial plate to the poles of the spindle. 
It follows metaphase and precedes telophase. 

Aneuploid — having a chromosome number that is not an exact multiple of the 
basic haploid number; heteroploid. 

Angiosperm — the flowering plants: a class having seeds enclosed in an ovary. 

399 



400 • GLOSSARY 

Anther — the pollen-bearing part of the stamen. 

Anthocyanin — any of a class of soluble glucoside pigments of flowers and plants; 
range in color from red through violet to blue. 

Apomixis — asexual reproduction in which the outward appearance of sexual repro- 
duction is retained but no fertilization occurs. 

Asexual — any mode of reproduction not involving fertilization, conjugation, or 
genetic recombination. Progeny have the same genotype as the parent. 

Autopolyploid — an organism having more than two homologous sets of chromo- 
somes in its somatic cells and derived from a single parent species. 

Autosome — chromosomes other than the sex chromosomes, ordinarily found in 
equal numbers in both males and females. 

Back-cross — the mating of a hybrid to one of the parental types used to produce 

the hybrid. 
Back mutation — the mutation of a mutant gene back to its original state. 
Balanced lethals — lethal genes so closely linked that crossing over is rare, the genes 

remain in repulsion, both homozygotes die, and only the heterozygote 

survives. 
Balanced polymorphism — two or more distinct types of individuals coexisting in 

the same breeding population, actively maintained by selection. 

Chiasma — a visible change in pairing affecting two out of the four chromatids in a 
tetrad or bivalent in the first meiotic prophase. The point of apparent 
exchange of partners is the chiasma. 

Chromatids — half chromosomes resulting from longitudinal duplication of a chro- 
mosome, observable during prophase and metaphase and becoming 
daughter chromosomes at anaphase. 

Chromosome — nucleoprotein bodies in the nucleus, usually constant in number for 
any given species, and bearing the genes in linear order. 

Cline — a geographical gradient in phenotypic traits. 

Clone — all the individuals descended from a single individual by asexual repro- 
duction. 

Coelom — the body cavity of most higher Metazoa; lined by a distinct epithelium. 

Coincidence — the ratio of observed double crossovers to expected double crossovers 
calculated on the basis of independent occurrence. This ratio is used as a 
measure of interference in crossing over. 

Crossing over — the exchange of corresponding segments between the chromatids of 
homologous chromosomes. The result is a recombination of genes between 
two homologous groups of linked genes. 

Cytology — the study of the structure, physiology, development, reproduction, and 
life history of cells. 

Deficiency — the absence or deletion of a segment of a chromosome. 

Deletion — a deficiency, especially in which an internal chromosomal segment is 
missing. 

Demographic transition — the change from a high birth rate — high death rate so- 
ciety to one with a low birth rate and a low death rate. 

Deuterostomia — animal groups in which the blastopore becomes the anus and the 
mouth is formed de novo. 

Differential reproduction — reproduction in which different types do not contribute 
to the next generation in proportion to their numbers. 



GLOSSARY • 401 

Diploid — having two sets of chromosomes. Somatic cells of higher plants and 
animals derived from the fertilized egg are ordinarily diploid in contrast 
to the haploid gametes. 

DNA — deoxyribonucleic acid, the hereditary material in the majority of species. 

Dominant — an inherited trait expressed in the phenotype, regardless of whether 
the gene controlling it is in the heterozygous or the homozygous condition. 
Thus the dominant trait from one parent is expressed in a hybrid but the 
recessive trait, though transmitted, is not expressed. Also a group of ani- 
mals or plants that is pre-eminent in a given region or at a given time. 

Doubling dose — the dose, usually of radiation, sufficient to cause a number of muta- 
tions equal to that occurring spontaneously. 

Duplication — the occurrence of a chromosome segment more than once in the same 
chromosome or haploid genome. 

Dysgenic — tending to be harmful to the hereditary qualities of a species. 

Ecological niche — the place occupied by a species in the community structure of 
which it is a part. 

Ecotype — an ecological race whose genotype is adapted to a particular restricted 
habitat as the result of natural selection. Many plant species have distinct 
ecotypes on the sea coast, in the desert, or in the mountains. 

Effective size of population — the number of individuals in a local breeding popu- 
lation that actually contribute genes to the next generation. 

Embryo sac — the mature female gametophyte in higher plants. 

Endosperm — the nutritive tissue, typically triploid, arising from double fertiliza- 
tion by the second male nucleus of two of the eight nuclei of the embryo sac. 

Enzyme — protein catalyst in living organisms, typically formed from a protein part 
(apoenzyme) conferring specificity and a nonprotein part (coenzyme) 
necessary for activity. 

Epigamic — promoting the union of gametes. 

Epistasis — the suppression of the expression of a gene or genes by other genes not 
allelic to the genes suppressed. Similar to dominance but involving the 
interaction of nonallelic genes. Sometimes used to refer to all nonallelic 
interactions. 

Ethology — the study of animal behavior. 

Euploid — an exact multiple of the haploid chromosome number. 

Eutheria — the placental mammals. 

Fertilization — the fusion of gametes to form a zygote. 

Finalism — the concept that the world is directed toward a definite purposive goal. 

Fitness — the number of offspring left by an individual as compared with the average 

of the population of which it is a member or compared to individuals of 

different genotypes. 
Flame bulb — a cup-shaped mass of protoplasm bearing a tuft of cilia projecting into 

the cavity of the cup, found at the closed inner end of a protonephridium. 
Founder principle — the concept that, when a small population invades a new area, 

evolutionary divergence may be hastened not only because of the new and 

probably different selection pressures but also because, due to sampling, 

the gene pool of this small group may differ in significant ways from that 

of the parental population. 

Gamete — a sex cell. 

Gametogenesis — the formation of gametes. 



402 • GLOSSARY 

Gametophyte — the gamete-forming haploid generation in higher plants. 

Gene — a Mendelian factor or unit of inheritance that occupies a fixed chromosomal 
locus, is transmitted in the germ cells, and, interacting with other genes, 
the cytoplasm, and the environment, controls the development of a char- 
acter. 

Gene flow — the spread of genes from one breeding population to others as the result 
of migration. 

Gene frequency — the proportion between one particular type of allele and the total 
of all alleles at this locus in a breeding population. 

Gene pool — the sum total of the genes in a given breeding population. 

Genetic drift — changes in gene frequency in small breeding populations due to 
random fluctuations. 

Genetic isolate — a breeding population not exchanging genes with any other group. 

Genetic system — the way in which the genetic material is organized and transmitted 
from one generation to the next. 

Genome — the chromosome complement of a gamete; also, of a zygote. 

Genotype — the entire genetic constitution of an organism. 

Gynandromorph — an individual with both male and female sectors; a sexual 
chimaera. 

Haploid — having only a single set of chromosomes. 

Hardy- Weinberg law — in a large random mating population in the absence of mu- 
tation and selection, gene frequencies remain constant. 

Hermaphrodite — an individual with functional ovaries and testes. 

Heterogametic — producing unlike gametes, especially with regard to the sex chro- 
mosomes. Where the male is XY, he is heterogametic. 

Heteromorphic — having more than one form. 

Heteroploid — having a chromosome number that is not an exact multiple of the 
basic haploid number; aneuploid. 

Heterosis — hybrid vigor. 

Heterozygous — having different alleles at one or more loci. 

Hexaploid — having six haploid sets of chromosomes. 

Homeostasis — a dynamic equilibrium in a biological system. 

Homologous — 1. similarity of structure due to similar hereditary and developmental 
origin; 2. chromosomes in which the same gene loci occur in the same 
sequence. 

Homozygous — having any specified gene or genes present in double dose so that 
the organism breeds true at these particular gene loci. 

Inbred — the result of matings between relatives. 

Incompatibility — the inability of pollen to fertilize due to failure of the pollen tube 
to grow normally in the style. 

Independent assortment — segregation of one factor pair occurring independently of 
the segregation of other factor pairs. 

Industrial melanism — the appearance of dark or melanistic forms of a species in 
industrial regions. 

Interference — the effect by which the occurrence of one cross-over reduces the prob- 
ability of another occurring in its vicinity. 

Interphase — the "resting" stage, used especially in referring to the phase between 
the two meiotic divisions. 

Intersex — an individual with traits intermediate between those of males and females. 



GLOSSARY • 403 

Introgressive hybridization — the addition of genes from one species to the gene 
pool of another species through hybridization and back-crossing. 

Inversion — rotation of a chromosome segment through 180 degrees so that the 
linear order of the genes is reversed relative to the rest of the chromosome. 

Isoalleles — alleles so similar in their effects that special techniques are needed to 
distinguish between them. 

Isolating mechanism — any intrinsic factor that prevents or reduces interbreeding be- 
tween two populations. 

Isomorphic — having similar form. 

Lamarckism — usually, the theory of the inheritance of acquired characteristics. 

Lethal — a gene or genotype that, when expressed, is fatal to its bearer. 

Linkage — the association of genes in inheritance due to their being on the same 

chromosome. Genes borne on homologous chromosomes belong to the 

same linkage group. 
Locus (pi., loci) — the position of a gene on a chromosome. 

Materialism — any theory that considers the nature of the universe to be sufficiently 

explained by the existence and nature of matter. 
Mean— the sum of a group of observations divided by the number in the group. 
Mechanist — one who regards the phenomena of nature as the effects of merely 

mechanical forces. 
Megaspore — the larger of the two kinds of haploid spores produced by hetero- 

sporous plants. In seed plants the megaspore gives rise to the embryo sac, 

the female gametophyte. 
Meiosis — the reduction divisions during which the chromosome number is reduced 

from diploid to haploid; two nuclear divisions during which the chromo- 
somes divide only once. 
Mendel's laws — segregation and independent assortment. 
Metabolism — the sum total of the chemical processes in living cells by which energy 

is provided, new materials assimilated or synthesized, and wastes removed. 
Metamorphosis — a more or less abrupt change in the form of an animal after the 

embryonic period. 
Metanephridia — nephridia (excretory organs) with open inner ends. 
Metaphase — the stage of nuclear division during which the chromosomes lie in the 

equatorial plane of the spindle; after prophase and prior to anaphase. 
Microspore — the smaller of the two kinds of haploid spores produced by hetero- 

sporous plants. In seed plants the microspore gives rise to the pollen grain, 

the male gametophyte. 
Mitosis — the process by which the nucleus is divided into two daughter nuclei, each 

with a chromosome complement similar to that of the original nucleus. 
Modifying factor — a gene that affects the expression of another nonallelic gene. 

Often without other known effects. 
Monohybrid — a cross involving parents that differ with respect to a single specific 

gene. 
Monosomic — a diploid with one chromosome missing from the chromosome com- 
plement. 
Multiple alleles — a series of more than two alternative forms of a gene at a single 

locus. 
Multiple factors — two or more pairs of factors with a similar or complementary 

cumulative effect on a single trait. 



404 • GLOSSARY 

Mutagenic — capable of inducing mutations. 

Mutation — in the broad sense, any sudden change in the hereditary material, includ- 
ing both "point" or gene mutations and chromosomal rearrangements. In 
the narrow sense, point mutations only. 

Mutation pressure — the continued recurrent production of a gene by mutation, tend- 
ing to increase its frequency. 

Mutation rate — the frequency with which a particular mutation occurs. Also the 
frequency of all mutations in a given population. 

Mutation rate gene — a gene that influences the mutation rate of genes at other loci. 

Nephridium — an excretory tubule. 

Normal curve — a symmetrical bell-shaped curve often approximated when fre- 
quency distributions are plotted from observations on biological materials. 

Octoploid — a polyploid with eight haploid sets of chromosomes. 

Oocyte — primary : egg mother cell giving rise by the first meiotic division to the 
secondary oocyte and the first polar body. The secondary oocyte at the 
second meiotic division gives rise to the ovum and to a second polar body. 

Oogonium — a cell giving rise to primary oocytes by mitosis. 

Orthogenesis — evolution more or less continuously in a single direction over a long 
span of time. Often used with vitalistic implications. , 

Orthoselection — natural selection acting continuously in the same direction over 
long periods of time. Often used in place of orthogenesis to avoid impli- 
cation of vitalism. 

Overdominance — the superiority of the heterozygote over both types of homo- 
zygotes. 

Paracentric — an inversion that does not include the centromere, but is entirely 

within one arm of the chromosome. 
Parthenogenesis — the development of a new individual from a germ cell (usually 

female) without fertilization. May be either haploid or diploid. 
Pericentric — an inversion that includes the centromere; hence both chromosome 

arms are involved. 
Phenocopy — environmentally induced nonhereditary phenotypic imitations of the 

effects of mutant genes. 
Phenotype — the sum total of the observable or measurable characteristics of an 

organism without reference to its genetic nature. 
Photosynthesis — the synthetic metabolism carried on by the chlorophyll-bearing 

tissues in plants. 
Phyletic evolution — evolution by a related group of species within a broad adaptive 

zone, carried on at moderate rates and without marked change of adap- 
tive type. 
Phylogeny — the evolutionary history of a taxonomic group. 

Pistil — in flowers, the female portion — the ovary, style, and stigma, collectively. 
Pleiotropic — a single gene influencing more than one character. 
Polar body — in oogenesis, the smaller cells produced during meiosis that do not 

develop into functional egg cells. 
Polygene — originally associated with a particular theory of quantitative inheritance 

but now frequently used as a synonym for multiple factor. 
Polymorphic — two or more recognizably different sorts of individuals within a 

single breeding population. 



GLOSSARY • 405 

Polyploid — an organism with more than two haploid sets of chromosomes. 
Polysaccharide — a molecule formed by the condensation of a number of simple 

sugar molecules — for example, starch, cellulose. 
Polytypic — generally, a species composed of several geographic races or subspecies. 
Position effect — change in the effect of a gene due to a change in its position with 

respect to other genes in the genotype as the result of chromosomal 

rearrangement. 
Preadaptation — a characteristic that enables an organism to be adapted to environ- 
mental conditions to which it has not yet been exposed. 
Preformation — the concept that the individual is present in miniature in the embryo 

and that development to adulthood involves growth but not differentiation. 
Prophase — the first stage of nuclear division. 
Protonephridia — nephridia with closed inner ends. 

Protostomia — those animal groups in which the blastopore becomes the mouth. 
Pseudoalleles — very closely linked genes, usually affecting the same trait, and 

showing a mutant phenotype rather than the wild type when in repulsion 

in heterozygotes. 
Pseudocoelom — a body cavity not lined with epithelial cells. 

Quantum evolution — relatively rapid evolution involving a major adaptive shift. 

Race — a subspecies or a geographical subdivision of a species. A geographically 
defined group of breeding populations that differs from other similar 
groups in the frequency of one or more genetically determined traits. 

Random mating — the situation when any individual of one sex has an equal prob- 
ability of mating with any individual of the opposite sex. 

Recapitulation — the theory that ontogeny recapitulates phylogeny; that is, that the 
development of the individual passes through phases resembling the adult 
forms of its successive ancestors. 

Recessive — an inherited trait only expressed in the phenotype when the allele con- 
trolling it is in the homozygous condition. Thus a recessive trait is not 
expressed in a hybrid. 

Reciprocal cross — a second cross similar to the first but with the sexes of the parents 
interchanged. 

Repeat — a duplication. 

Reproductive isolation — inherent blocks to crosses between members of different 
breeding populations. 

Roentgen (r) — the unit of measurement of dosage for ionizing radiation. Equal to 
the amount of radiation that in air at STP will produce 2.1 X 10 9 ion 
pairs per cubic centimeter or in tissue approximately two ionizations per 
cubic micron. 

Saprophyte — any organism living on dead or decaying organic material. 

Segmental allopolyploid — an allopolyploid in which some chromosome segments 

from the parent species are still homologous. 
Segregation — the separation of maternal from paternal chromosomes at meiosis 

and hence the basis for Mendel's first law. 
Semilethal — a gene or genotype that, when expressed, reduces the viability of its 

bearers to less than half of that of the "normal" or standard type. 
Serology — the study through antigen-antibody reactions of the nature and specificity 

of antigenic materials from different sources. 



406 • GLOSSARY 

Sex chromosomes — chromosomes that are particularly involved in sex determination. 
Sex reversal — a change in the sexual character of an individual from male to female 

or vice versa. 
Sexual — a mode of reproduction normally involving of fusion of gametes and 

genetic recombination. 
Sexual isolation — reproductive isolation due to a tendency toward homogamic 

mating. 
Sexual selection — selection based on male competition or female choice and respon- 
sible for sexual dimorphism. 
Solenocyte — a long tubular cell with a flagellum at the base of the tube that extends 

into the tube and forms the closed end of a protonephridial tubule. 
Somatic — referring to the body tissues, as contrasted with the germinal tissues that 

give rise to the germ cells. 
Speciation — the process by which new species are formed. In the restricted sense, 

the splitting of one species into a number of different contemporaneous 

species. 
Spermatid — the haploid cell that results from meiosis and develops into a functional 

spermatozoan without further nuclear division. 
Spermatocyte — primary: a sperm mother cell giving rise by the first meiotic divi- 
sion to two secondary spermatocytes. The secondary spermatocytes at the 

second meiotic division give rise to four haploid spermatids. 
Spermatogonium — a cell giving rise to primary spermatocytes by mitosis. 
Spontaneous generation — the direct formation of living organisms from nonliving 

matter. 
Sporophyte — the spore-forming diploid generation in higher plants. 
Stamen — in flowers, the male portion — the anther containing the pollen plus the 

filament or stalk. 
Standard deviation — the square root of the sum of the deviations from the mean 

squared and divided by one less than the number of observations. A 

measure of the variability of a population of individuals. 
Standard error — the standard deviation divided by the square root of the number 

of observations. A measure of the variation of a population of means. 
Subspecies — see Race. 
Subvital — a gene or genotype that, when expressed, reduces the viability of its 

bearers significantly below that of the "normal" or standard type but has 

a viability at least half as great. 
Supervital — a gene or genotype that, when expressed, is significantly more viable 

than the "normal" or standard type. 
Sympatric — coexisting in the same area, with the implication that crossing is at least 

possible. 
Synapsis — the pairing of homologous chromosomes of maternal and paternal origin 

during the first meiotic prophase. Also observed occasionally in somatic 

cells — for example, salivary gland chromosomes in Drosophila. 
Systematics — taxonomy. The classification of organisms. 
Systemic mutation — mutations of major effect presumed to give rise to new species 

or higher categories at a single step. 

Teleology — the concept that evolution is purposeful and is directed toward some 

definite goal. 
Telophase — the last phase of nuclear division, following anaphase, during which 

the daughter nuclei are formed and separate cells are formed. 



GLOSSARY • 407 

Test cross — a cross between a presumed heterozygote and a recessive homozygote. 
Tetraploid — a polyploid with four haploid sets of chromosomes. 
Transduction — genetic recombination in bacteria mediated by bacteriophage. 
Transformation— genetic recombination in bacteria brought about by the addition 

of DNA from a different strain to the culture. 
Transient polymorphism — temporary polymorphism observed while one adaptive 

type is replacing another. 
Translocation — change in position of a chromosome segment to another part of the 

same chromosome or to a different chromosome. Reciprocal — the exchange 

of segments between two chromosomes. 
Triploid — a polyploid with three haploid sets of chromosomes. 
Trisomic — an organism, otherwise diploid, that has three chromosomes of one type. 

Variance — the mean squared deviation from the mean. The square of the standard 

deviation. 
Vitalism — the concept that living organisms are animated by a vital principle or 

force distinct from physical forces. 

Wild type — the customary phenotype. Also the most frequent allele in wild popu- 
lations. 

Zygote — the cell produced at fertilization by the union of gametes. Also the indi- 
vidual derived from this cell. 



Index 



Index 



ABO blood groups, 178, 265, 342f. 

Acanthocephala, 132f. 

Acoela, 131 

Actinopterygii, 44 

Adalia bipunctata, 254 

adaptation, 3ff., 15, 113ff., 239f., 301, 

303f.; individual, 4, 245; population, 

5, 245 
adaptive behavior, 8, 10, 12 
adaptive neutrality, 250 
adaptive radiation, 42 
adenosine triphosphate (ATP), 64f., 104 
Agassiz, L., 32, 42 
Agelaius phoeniceus, 269 
age of earth, 41, 5 If. 
Age of Fishes, 42 
Age of Mammals, 42 
Age of Reptiles, 42 
age of universe, 5 iff. 



Agnatha, 44 
agriculture, 348 
albinism, 172f. 
algae, l44ff. 
allantois, 46 
allesthetic traits, 3l6f. 
allopatric, 270 

allopolyploidy, 157, 205, 313 
alternation of generations, 188f. 
Ambystoma, 92 
American Indians, 265 
amino acid synthesis, 62 
amnion, 46, 92 
amniotes, 92, 120 
amoebae, 125 
amphibians, 44f., 92, 110 
Amphineura, 135 
Amph'ioxus, 142 
amphiploidy, 157, 205 
analogy, 95ff., 103 



411 



412 • INDEX 



anaphase, 186 
Anaxagoras, 15 
Anaximander, I4f., 57 
Ancon sheep, 207 
aneuploidy, 204 
Angiospermae, I44f., 152 
anisogametes, 306 
Annelida, 134, 136 
anthropoid apes, 327 
Anthropoidea, 325ff. 
antibiotics, 242 

antigen-antibody reactions, HOf. 
aortic arches, 88 
apomixis, 314 
aposematic coloration, 10 
Aquinas, St. Thomas, l6f. 
Arachnida, 111, 139 
archetype, 95 

Aristotle, 15f., 18, 23, 57, 80, 83 
Arrhenius, 59 
Arthropoda, 132, 136, 139 
artificial selection, 158, 24 If. 
Aschelminthes, 133 
asexuality, 303f., 3l4f. 
astaxanthin, 107, 109 
atomic theory, 16 
Auerbach, 35 
Augustine, St., 16 
Australian, 69 
Australoid, 343, 345 
Australopithecus, 335, 338 
autocatalysis, 64, 66 
autopolyploidy, 204 
autosome, 191 
autotrophic, 66, 148 
Aves, 46 

A vitamins, 107ff. 
axolotl, 92 

B 

back cross, 173 

Bacon, Sir Francis, 18 

bacteria, l44f., I48f. 

bacteriophage, 244, 302 

balanced lethals, 227, 256, 313 

balanced polymorphism, 250, 254ff., 342 



balance theory of sex determination, 308 

Baldwin effect, 244f. 

Bar eye, 203 

barnacle, 58 

barriers, 269 

Bateson, W., 34, 158, 195, 211, 253 

Beagle, 26f. 

Bennettitales, 152 

binocular vision, 325 

binomial, 236 

binomial system, 19, 81ff. 

biogenetic law, 87 

biogeographical realms, 69ff. 

biological success, 10 

bipedal locomotion, 330, 335 

bipinnaria larva, 140 

birds, 46 

Biscutella laevigata, 282 

bisexual species, 306 

Bis ton betularia, 251 

blastaea, 130 

blastopore, 132 

blastula, 88, 130 

blue babies, 88 

blue-green algae, l44ff. 

Blyth, E., 23 

Bohr effect, 121 

Bonellia, 306f. 

Botallus, duct of, 89 

Boyden, A. A., Ill 

brachiation, 327, 329 

Brachiopoda, 135, 140 

brachyury, 180 

Bridges, C. B., 308 

Bronze Age, 345, 348 

Broom, R., 335 

brown algae, 145, 148 

Bryophyta, I44f., 148, 150f. 

Bryozoa, 134 

Buffon, G. L. L. de, 17, 19, 21, 23 

Bufo, 287 

Bushmen, 343, 345 



Carnivora, 80 
carotenoids, 106f., 146 



INDEX • 413 



cataclysmic evolution, 284 

catarrhine, 327, 329 

Caucasoid, 343 

cellular fusion, 302 

Cenozoic, 4 If. 

centromere, 190 

Cercopithecidae, 324, 327, 335 

cerebrum, 327 

cervical vertebrae, 97f. 

Cesalpino, 17 

Chaetognatha, 139 

chain of being, 15, 18f., 23 

Chambers, R., 23 

chemical evolution, 59ff. 

chemical mutagens, 209 

chiasmata, 190, 314 

chimpanzee, 329 

chlorophyll, 105 

Chlorophyta, 145, 148, 150f. 

Choanichthyes, 44 

choanoflagellates, 128 

Chondrichthyes, 44 

Chordata, 90, 132, 136, 139, I4lf. 

chromatid, 186, 190 

chromatophores, 7 

chromosome, 186, 188, 190, 303 

chromosome homology, 159 

chromosome map, 197 

chromosome rearrangements, 199ff. 

chrysomonads, 125 

Chrysophyta, l45f., 150 

Ciliata, 125 

cinquefoil, 228, 230, 272 

cis-trans, 204 

Clausen, Keck, and Hiesey, 272 

classification of plants, l44f. 

climate and evolution theory, 75 

cline, 271 

clover, 182f. 

club mosses, I44f., 152 

coadaptation, 258 

Coelenterata, 128ff. 

coelom, 134ff. 

coincidence, 198 

colchicine, 158, 204 

comb jellies, 129 



comparative anatomy, 19, 22, 95fF. 

competition, 6, 240 

conifers, 145, 152 

conjugation, 306 

Continental Drift, 75 

continental islands, 75f. 

continuous variation, 2l6ff. 

convergent evolution, 97 

cooperation, 6, 241 

corn, 211, 218, 220 

Correns, C, 33 

cosmology, 5 Iff. 

cosmozoa, 59 

countershading, 8, 10 

coupled reaction, 64 

Cro-Magnon man, 338ff. 

crossing over, 195ff. 

Crossopterygii, 44 

crossveinless condition, 244f. 

Crustacea, 93, 139 

cryptic coloration, 7, 10, 12, 318 

cryptomonads, I45f., 150 

Ctenophora, 129, 131 

cultural evolution, 345ff. 

Cuvier, G., 22f., 42, 95 

cyanide, 182f. 

Cyanophyta, I45f. 

Cycadofilicales, 152 

Cynips, 270 

D 

Dart, R., 335 

Darwin, Charles, 20, 23, 25fT., 69, 83, 

95, 158, 164, 166, 239 
Darwin, Erasmus, 21, 23 
Darwin, Robert, 25f. 
Datura, 229 
da Vinci, Leonardo, 17 
DDT, 242 
deficiency, 199 
deletion, 199 
De Maillet, 20 
de Maupertius, 19 
Democritus, 16 

demographic transition, 364, 367f. 
demography, 363ff. 



414 • INDEX 



deoxyribonucleic acid (DNA), 66, 150, 

I60f., 302 
Descartes, R., 18, 57 
Deuterostomia, 132, 135, 139f. 
de Vries, H., 33, 213f. 
developmental homeostasis, 258 
diabetes, 260 
diatoms, I45f. 

differential reproduction, 239 
dihybrid, 175 
dinoflagellates, I45f., 150 
dinosaurs, 46 
dipleurula larva, 140 
diploid, 188 
diploidy, 304f. 
Dipnoi, 44, 118 
Diptera, 101, 295 
discontinuous traits, 216 
disruptive coloration, 7 
distribution of species, 268f. 
Dobzhansky, Th., 289, 292 
domestication, 158f. 
dominance, 169, 252ff. 
dominance theory of heterosis, 221 
Doppler effect, 53 
double fertilization, 190 
doubling dose, 357 
Drosophila, 100f., 196, 202, 226, 228f., 

249, 255, 257ff., 288, 293, 308ff., 312, 

353 
Dryopithecus, 332, 335 
Dubinin, 259 
Dubois, 336 
duplication, 199, 201 



East, E. M., 218, 223 
Echinodermata, 111, 139ff. 
Echiurida, 134, 137 
ecological isolation, 286 
ecological niche, 10, 42, 68 
ecotype, 228 
Ectoprocta, 134f., 140 
effective population size, 264f. 
elasmobranchs, 117, 120 
elements, 60 



embryo culture, 288 
embryo sac, 190 
Embryophyta, 150f. 
emigration, 367 
Empedocles, 15 
Encyclopedists, 17 
endosperm, 190 
Entoprocta, 134 
environment, 5ff. 
Epicurus, 16 
epigamic, 317 
epistasis, 182 
Equidae, 46ff., 297 
Escherichia coli, 302 
Ethiopian, 69, 71 
ethology, 287, 318 
eugenics, 376ff. 
Euglena, 107, 124 
Euglenophyta, 145, 148, 150 
Eumycophyta, 145, 149 
Eutheria, 46, 323 
evolving universe, 53ff. 
excretion, 115ff. 
eye, 96f., 325 



Felidae, 80 

Felis, 80, 100 

female choice, 3l6f. 

fermentation, 65 

ferns, 145, 152 

finalism, 43 

Fisher, R. A., 35, 163, 223, 253 

Flagellata, 124ff., 150 

flatworms, 129ff. 

Flemming, 33 

flowering plants, 145, 152 

fossil record, 330fT. 

founder principle, 271 

freemartin, 311 

fungi, l44ff. 



galaxies, 53, 55 
Galen, 16 



INDEX • 415 



Gale op sis, 231, 282 

Galton, 34 

gametogenesis, 188 

gametophyte, 189 

Gamow, G., 55 

gastraea theory, 130 

Gastrotricha, 134 

gastrula, 88, 130 

Gegenbauer, K., 32 

gene, 171 

gene flow, 237, 279 

gene frequency, 235ff. 

gene homology, 100f., 159f. 

gene pool, 270, 298 

generalized forms, 43f. 

genetic drift, 237, 263ff. 

genetic homeostasis, 258 

genetic recombination, 301n\, 313ff. 

genetic systems, 301 ff. 

genotype, 172; frequency of, 235f. 

geological column, 40f. 

Gephyrea, 134 

germ line theory, 33 

gibbon, 327 

Giles, 212 

gill arches, 90 

glaciation, 73 

golden brown algae, l45f. 

Goldschmidt, R. B. G., 101 

Gondwana, 75 

goose tree legend, 58 

gorilla, 327, 329 

grackles, 84 

graptolites, 142 

Gray, A., 30, 32 

green algae, 145, 148, 151 

guinea pig, 100 

Gymnospermae, 145, 152 

gynandromorph, 310 



Hardy-Weinberg, 33, 164, 235ff., 250 

Harvey, W., 17f., 57 

Hemichordata, 111, 14 If. 

Hemizonia angustifolia, 272 

Henslow, J. S., 26 

heredity vs. environment, 168 

hermaphroditism, 306, 313f. 

heterogametic, 307f. 

heteromorphic, 305 

heteroploidy, 204 

heterosis, 155, 220ff., 256ff. 

heterotrophic, 66, I48f. 

heterozygous, 171 

heterozygous sporophyte, 308 

Holarctic, 69, 73, 75 

homeostasis, 7 

homeotic mutants, 101 

Hominidae, 327ff. 

Hominoidea 327ff. 

Homo, 336ff. 

homology, 95ff., 103 

homozygous, 171 

honey bee, 310 

Hooker, Sir Joseph, 29, 32 

horn worts, 145 

horseshoe crab, 111 

horsetails, 145, 152 

Hutton, J., 20, 23 

Huxley, T. H., 32 

hybrid, 170 

hybrid breakdown, 288 

hybrid inviability, 288 

hybrid sterility, 155, 288 

hybridization, 155ff., 277ff. 

Hylobates, 332 

Hylocichla, 80 

Hylodes, 91 

hyoid, 90 

hyomandibular, 90 



H 

Haeckel, E., 32, 87f., 93, 130 
Haldane, J. B. S., 35, 163, 254 
Haldane's rule, 288 
haploid, 188 
haploidy, 304f. 



immunology, 11 Of. 
inbreeding, 221, 223 
incompatibility, 255 
independent assortment, 173, 193 
induced mutations, 209f., 353ff. 



416 • INDEX 



industrial melanism, 2 5 Off. 

inheritance of acquired characteristics, 

15, 21f. 
Insectivora, 323 
insemination reaction, 288 
interaction between genes, 180ff. 
interference, 198, 314 
intersex, 31 If. 

introgressive hybridization, 280 
inversion, 201, 229, 255, 257ff., 314 
ionic composition, 113ff. 
Iris, 280 
Irish elk, 43 
Iron Age, 345, 348 
isoalleles, 179 
isogametes, 306 
isolation, 27 Iff., 279, 286 
isomorphic, 305 



J 



Java man, 336 
Jimson weed, 229 
Johannsen, W. L., 34, 246 
Jones, D. F., 221 
Junonia, 84 



Kant, Immanuel, 18 

Kinorhyncha, 133 

Klinefelter's syndrome, 284, 309 



Lagomorpha, 111 

Lamarck, 2 Iff., 34, 95 

Lamarckianism, 244f. 

Laurasia, 75 

Leakey, L. S. B., 335 

Leibnitz, Gottfried Wilhelm, 18 

lemurs, 324, 330 

leopard frog, 6ff., 84, 274, 288 

lethals, 226f., 246 

leukemia, 358 

life cycle, 188f. 

Limnopithecus, 332 

Limulus, 111 



linear order of genes, 196ff. 
Lingula, 135 
linkage, 193, 195ff., 313 
Linnaeus, 19, 81, 83 
liverworts, I44f., 151 
lobe-finned fish, 44, 49 
Lorisiformes, 324f., 330 
Lucretius, 16 
lung fish, 44 
Lycopsida, 145, 152 
Lyell, C, 20, 29, 32 
Lymantria, 312 
Lysenko, T. D., 22 

M 

macroevolution, 294f. 

macula lutea, 325 

malaria, 257 

male competition, 316 

male haploidy, 309 

Malthus, T., 19, 27, 360f., 365f. 

mammals, 46, 323 

man, 12, 92, 101, 323ff. 

marmosets, 327 

marsupials, 46 

Mastigophora, 124 

materialism, 16 

Matthew, P., 23 

Matthew, W. D., 75 

Mayr, E., 271, 292, 336 

mean, 216 

medicine, practice of, 352f. 

megaevolution, 294f. 

Megalopa, 92 

megaspores, 190 

meiosis, 190, 303 f. 

meiotic drive, 255 

Melandrium, 309 

Mendel, G., 33, 164, l66ff., 185 

Mendelian population, 270, 292 

Mesolithic, 345, 348 

Mesozoa, 128 

Mesozoic, 42 

metamorphosis, 10, 91 f., 121 

metaphase, 186 

Metatheria, AG 



INDEX • 417 



Metazoa, 124f., 128ff. 

Michurinism, 22 

microspores, 189f. 

middle ear ossicles, 90 

migration, 237, 278ff. 

mimicry, llf., 259, 318 

mink, coat color, 180 

mitosis, 185ff., 303f. 

modern synthesis, 35 

modifying factors, 220 

Mollusca, 132, 134ff. 

Mongoloid, 343 

monohybrid, 171 

monosomic, 204 

monotremes, 46 

Moody, 111 

Moore, J. A., 274 

Morgan, T. H., 34 

mosses, 145, 151 

mouse, brachyury in, 180 

mule, 155 

Muller, H. J., 35, 254, 289, 353 

multiple alleles, 177ff. 

multiple factors, 218ff. 

mutation, 66, 207ff., 237f., 247, 350ff.; 

rates of, 21 Off., 237f., 353 
mutation pressure, 237f. 
mutation rate genes, 210 
mutation theory, 33, 213 
Myxomycophyta, 145, 149 

N 

natural philosophers, 17f. 

natural selection, 15, 23, 31, 96, 237, 

239ff., 266, 316 
natural system of classification, 80 
nature of the universe, 53ff. 
Nauplius, 93 
Neanderthal man, 336ff. 
Nearctic, 69f. 
Needham, J. T., 59 
Negroid, 343 
Nematoda, 133 
Nematomorpha, 133 
Nemertea, 132 
Neo-Darwinism, 35 



Neolithic, 345, 348 
Ne optima, 136 
Neotropical, 69f., 72 
Newton, Sir Isaac, 57 
New World monkeys, 327 
Nilsson-Ehle, 218 
nitrogen excretion, 120ff. 
normal curve, 2l6f. 
notochord, 90, 142 
Nuttall, G. H. F., Ill 

O 

oceanic islands, 75ff. 
Oenothera, 33, 203, 213f. 
Olduvai Gorge, 333 
Old World monkeys, 327ff. 
ontogeny, 87 
Onycophora, 137ff. 
oogenesis, 188 
Oparin, A. I., 62 
orangutan, 327 
Oreopithecus, 335 
organic compounds, 6 Iff. 
Oriental, 69, 71 
orthogenesis, 43 
orthoselection, 43 
osmosis, H4ff. 
Osteichthyes, 44 
ostracoderms, 44 
overdominance, 221, 223, 256 
overpopulation, 366ff. 
Owen, R., 32 



paedogenesis, 92 
Palearctic, 69f. 
Paleolithic, 345fT. 
Paleozoic, 42 
pangenesis, 33 
Panther a, 80, 100 
Paracelsus, 57 
paracentric, 201 
Paranthropus, 335 
Parazoa, 128 
parthenogenesis, 314 



418 • INDEX 



Pasteur, L., 59 

Pearson, K., 34 

Peking man, 336 

perfecting principle, 15 

pericentric, 201 

Peripatus, 138 

Peromyscus, 220, 286 

Phaeophyta, 145, 148 

phenocopy, 244 

phenotype, 172 

phenylthiocarbamide (PTC), 235 

Philo sophie Zoologique, 22 

Phoronida, 135 

phosphorylation, 64 

photoreceptors, 106ff. 

photosynthesis, 62, 65f., 105 

phyletic evolution, 294 

phylogeny, 83, 87f. 

physiological isolation, 287f. 

Phytomonadina, 306 

Piltdown man, 340 

Pithecanthropus, 336fT. 

placenta, 92 

placental mammals, 46 

Placodermi, 44, 49 

planula larva, 128, 131 

Platanus, 293 

Platyhelminthes, 129 

platyrrhine, 327 

pleiotropic, 209 

Pliny, 16 

Pliopithecus, 332 

Plunkett, 254 

Pneumococcus, 212, 302 

Pogonophora, 139f. 

pollex, 100 

polygenes, 220, 245 

polymorphism, 81, 249ff., 270 

Polynesian, 343, 345 

polyploidy, 157f., 204f., 229ff., 281ff. 

polytypic, 81, 250, 270, 34lff. 

Pongidae, 327 

population structure, 268ff. 

Porifera, 125, 127 

porphyropsin, 108fT. 

position effect, 203f. 



Potentilla glandulosa, 228, 272, 274 

preadaptation, 296ff. 

precipitin test, 111 

prehuman, 332f. 

Priapulida, 133 

Primates, 111, 323ff. 

primitive, 44 

Primula vulgaris, 255 

Proconsul, 332 

prophase, 186 

Propliopithecus, 332 

prosimians, 323ff. 

Protheria, 46 

Protista, 150 

Protostomia, 132 

Protozoa, 124f., 150 

pseudoallelism, 203f. 

pseudocoel, 132f. 

Psilophy tales, 15 If. 

Psilopsida, 145, 152 

pterodactyl, 97 

Pteropsida, 145, 152 

Punnett, R. C, 195, 211, 253 

Pyrrophyta, I45f., 150 



quantum evolution, 294ff. 
Quiscalus, 84 

R 

race, 81, 250, 268ff., 34lff. 

radiation, 209, 350ff. 

Rana pipiens, 6ff., 84, 274, 288 

random mating, 235 

Rapbanobrassica, 205, 220 

Rassenkreis, 84 

rates of evolution, 42f. 

Ray, J., 19 

rearrangements, 199ff. 

recapitulation, 87, 90, 121, 140 

recessive traits, 169 

recombination, 177ff., 301f., 313ff. 

Redi, F., 59 

relict populations, 73 

Renaissance, 17 



INDEX • 419 



reproductive isolation, 286ff. 

reptiles, 45f. 

resistant strains, 242, 244 

respiration, 65 

reverse mutations, 237 

Rhodophyta, l45f. 

rhodopsin, 108ff. 

ribonucleic acid (RNA), 150, l60f., 301 

Richter, 59 

ring of races, 84 

Robinson, 335 

Rotifera, 134 

roundworms, 133 



St. Hilaire, 2 Iff., 95 

salivary gland chromosomes, 159 

sampling, 263f. 

Santa Gertrudis cattle, 241 

Sarcodina, 125, 150 

Scala naturae, 15 

Schizomycophyta, 145, 148 

schizophrenia, 260 

Scholasticism, 17f. 

seasonal isolation, 287 

Sedgwick, A., 26 

seed ferns, 152 

segmental allopolyploid, 205 

segmentation, 100f., 136f. 

segregation, I68ff., 193 

selection coefficient, 245ff., 256 

self-duplication, 66, 161 

self-fertilization, 314 

self -sterility, 255, 313 

serial homology, 100 

sex chromosomes, 191 

sex determination, 191, 306ff. 

sex linkage, 191f. 

sex reversal, 31 Of. 

sexual differentiation, 310ff. 

sexual dimorphism, 316 

sexual isolation, 287 

sexual reproduction, 183, 303 

sexual selection, 239, 298, 315ff. 

sexuality, 303f. 

Shapley, H., 66 



sickle cell anemia, 257, 342 

Simpson, 34 

Sinanthropus, 336 

Sipunculida, 134, 137 

Sivapithecus, 332 

slime molds, 145, 149 

Smith, W., 20, 23 

snapdragon, 180 

Solarium, 229f. 

Spallanzani, L., 59 

Special Creation, 17 

specialization, 43f. 

speciation, 42, 268, 29lff. 

species, definitions, 29lff. 

species concept, 19, 79ff. 

Spencer, R, 24 

spermatogenesis, 188 

Sphenopsida, 145, 152 

spontaneous generation, I4f., 57, 59 

sporophyte, 189 

Sporozoa, 125 

standard deviation, 2l6f. 

standard error of the mean, 217 

standard error of a ratio, 264 

steady state universe, 53, 55 

Strasburger, E., 33, 185 

strontium-90, 357f. 

Sturnella, 287 

Suarez, 17 

subspecies, 81, 268ff. 

successive creation, 22 

Suctoria, 125 

symmetry, 6 

sympatric, 270 

syngamy, 306 

Sy sterna Naturae, 19, 81 

systematics, 19, 32, 79ff. 

systemic mutation, 295f. 



tarsiers, 325 
taxonomy, 79ff. 
Tchetverikov, 35 
teleology, 16 
telophase, 186 
terrestrial life, 117ff. 



420 • INDEX 



test cross, 172 

Thales, 57 

thrushes, 80 

T locus, 255 

tool tradition, 347f. 

tornaria larva, 140 

Tracheophyta, 145, 15 Iff. 

Tradescantia, 287 

transduction, 212, 302 

transformation, 160, 212, 302 

transient polymorphism, 250ff. 

translocation, 202, 229, 314 

tree shrews, 324 

Trijolium pratense, 255 

trisomic, 204 

trochophore larva, 132, 134ff., 140 

true fungi, 145, 149 

Turner's syndrome, 309 

type concept, 83 



vitalism, 15, 43, 59 
Voltaire, 19 
von Baer, K. E., 32, 88 
von Helmholtz, H. L. F., 59 
von Tschermak, E., 33 

W 

Waddington, 244 
Wallace, A. R., 24, 29f. 
Weismann, A., 33, 191 
Wells, W., 23 
whales, 111 
wheat rust, 244 
Wilberforce, S., 32 
Wilhelm, 111 
Wolff, K. F., 19 
Wright, S., 35, 163, 253 



U 
Ussher, J., 51 



Xenophanes, 15, 57 



van Helmont, 57 

variance, 217 

vascular plants, 145, 148, 151ff., 157 

vertebrates, 44f{., 143 

vestigial organs, lOlf. 

viruses, 150, 161, 302 



yellow-green algae, l45f. 



Zinjanthropus, 335, 347 
Zoonom'ta, 21 
zygotic, 87 f. 
zygotic selection, 245 



MARSTON SCIENCE LIBRARY 





Date Due 




Due 


Returned 


Due 


Returned 


|ftV 6 19* 


^OCT 24 


1996 




OCT 27 1997 


OCT 2 01997 







































































































































































UNIVERSITY OF FLORIDA 



3 1262 05585 6784 









s* / 



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